Jeremy McGrath was a dominant 125SX rider in the West Region in 1991 and '92, riding a Peak/Pro Circuit Honda CR125. But in '92, when the series was East, Mitch Payton and Team Honda let Jeremy ride the 250 class. In five starts the future King of Supercross was in the top ten every time. In '93 he moved up and won the third round at Anaheim. The rest is supercross history.

The man who would usurp Jeremy McGrath at the top, Ricky Carmichael, had a rougher transition than the King. He would ride the 125 class outdoors in '99, and the 250 class full-time in AMA Supercross. That meant outdoors on the 125 in '98, moving up to the 250 indoors in '99, then back down to the 125 that summer. Carmichael was dominant on the 125, but a bit of a mess on the 250. He crashed out of the second-round San Diego SX and missed several races, and once he returned, he never got to the podium. All told it would take RC a total of 21 250SX main events before he finally got his first win at Daytona '00. One year later he won 13 straight and his first of five AMA Supercross Championships.   

In 2002 Chad Reed moved to America full time to ride for Yamaha of Troy. This after a season on the 250 Grand Prix circuit of the FIM Motocross World Championship in Europe, where he battled often with Stefan Everts. In the states Reed was slotted to ride the 125SX East on a YZ250F, but Yamaha allowed him to first ride three 250SX races out west on a Y250 two-stroke. The Australian went 6-5-8 in his three starts, then dropped down and dominated the 125 East, winning all but the last round at the Pontiac Silverdome. The next year he started out AMA Supercross with a win at the Anaheim opener. He would win his first AMA Supercross Championship just one year later. 

Damon Bradshaw actually rode a 250SX before he ever lined up on a 125. In his rookie season of 1989, Yamaha placed him in the 125 East Region, but allowed him to start the season in the 250 class out west. He crashed himself out in his heat race at Anaheim and was forced to sit out the Seattle doubleheader that followed. But he lined up for the '89 San Diego round and finished an impressive third behind only Rick Johnson and Jeff Stanton. Then Bradshaw dropped down to the 125 and won the East Region, and then narrowly lost the 125 Pro Motocross title to Mike Kiedrowski. One year later Bradshaw opened the AMA Supercross series with wins in both Anaheim and Houston.

Back in the day, before there was a 125 class in AMA Supercross, top 125cc riders like Broc Glover and Johnny O'Mara were brought along slowly when it came to racing in stadiums. Team Yamaha's Glover was only allowed to ride limited 250SX races, and the future AMA Motorcycle Hall of Famer was only allowed to race four SX rounds from 1976 until 1980, during which time he won three straight 125 AMA Pro Motocross Championships. Johnny O'Mara turned pro in 1980 on a Mugen Honda but was not allowed to race AMA Supercross. When he signed with Team Honda proper the following year, he was allowed to ride 250SX. And when O'Mara went to Europe in the fall of '81 for the Trophee and Motocross des Nations, it would mark the first time that he ever raced outdoors aboard a 250 or a 500. O'Mara came through just fine, and Team USA swept both races.

Speaking of Team USA, back in 2009, manager Roger De Coster, then with Rockstar/Makita Suzuki, needed a second 450 rider as James Stewart was out with an injury. He tapped his own 250-class rider in Ryan Dungey, who had raced 250s outdoors up to that point, though he did do three 450SX rounds in '08. Dungey ended up winning his class and helping Team USA get an unlikely win. The following year (2010) he moved up to the 450 full-time and won both crowns—450 SX and 450 MX.

James Stewart smashed every record there was in the 125 class aboard his Kawasaki KX125. He then rode a single race aboard a KXF250, the last round of the '04 AMA Pro Motocross Championship at Glen Helen and was even more dominant. He then went to the 250 class for the start of the '05 AMA Supercross Championship, but broke his arm at the second round (Phoenix). He was back two months later and ended up winning what turned out to be only his third AMA Supercross race in the premier class, Dallas '05.

Like Stewart, Ryan Villopoto focused entirely on the smaller class until it was time to move up. He won three straight AMA 250 Pro Motocross Championships (2006-'08) then went to the 450 in '09. He showed potential right away, even winning his hometown Seattle race—his first in the class. He won the opening 450 National at Glen Helen too and seemed like he was going to contend for the title, only to injure his knee before the second round at Hangtown. He tried to ride through it and finished 16th in the first moto but then dropped out. When he returned to start the '10 Monster Energy AMA Supercross Championship, RV was fully used to the 450 and ended up winning several rounds as he battled Ryan Dungey for the title. But then RV crashed hard at St. Louis and suffered a broken leg. He missed the entire season of 450 MX. But 2011 would see him finally arrive, as he swept both 450 SX and MX, and went on to win four straight AMA Supercross Championships. All told, Villopoto would end up with nine major titles, six of them on the Monster Energy Kawasaki 450.

Eli Tomac dabbled his toe in the 450 waters during the 2013 SX season while he was on his way to winning the 250SX West Region. GEICO Honda let Tomac do four rides when the series was back east, and his high finish was fourth at Daytona. He moved up full time in 2014 but struggled at first—he even missed the start of the season with an injury. He ended up winning a single 450 National that summer, and then got himself prepared for a proper start in '15. After a last-place finish in the season-opener at Anaheim, he won his first 450SX main event one week later at Phoenix. But the 450 SX title would not come easy for Eli. In fact, he waited until 2020 before he finally got the job done. 

Cooper Webb stayed on 250s exclusively until he made the move up in 2017. By that point he was a two-time 250SX West Region Champion as well as a 250 Pro Motocross Champion. But for some reason he didn't mesh right away with the 450. He went through all of 2017 and '18 without winning a single race in both SX and MX. Then he switched to the Red Bull KTM team and won his first 450SX in only his third start with orange. He went on to win two AMA Supercross titles (and 21 450SX main events) and is presently tied with the aforementioned Tomac for the '23 series points lead after 11 rounds.

Jeff Emig had a weird transition. Back in the early ‘90s he was Yamaha’s top 125 rider, winning the ’92 AMA 125 Pro Motocross Championship. By that point he was already dabbling in 250SX, racing five events in ’91, then racing all of the ’92 AMA Supercross Championship in the 250 class. Outdoors, Emig dropped back down to a YZ125 and won the title. And then things got strange. Emig spent both the ’93 and ’94 seasons riding a 250 in AMA Supercross and the 125 outdoors. He did not win a single 250SX, nor did he win another 125 title. Finally, in 1995 Yamaha made Emig a full-time 250 rider. He won the controversial ’95 Las Vegas SX, and then the ’95 Southwick 250 National, his first two successes on the bigger bike. For ’96 Emig left Yamaha for Kawasaki and ended up winning the next two 250 Pro Motocross titles as well as the ’97 AMA Supercross Championship.

Finally, there's Kevin Windham. In the mid-nineties Yamaha signed Windham to ride the 125 class, but they allowed him to moonlight every now and then on the 250 in SX when the races were on the opposite coast. He finished on the podium in Charlotte in '96 while on his way to his first 125SX West Region title. One year later he came back to Charlotte and ended up winning, making Kevin Windham the first rider ever to win a premier-class AMA Supercross while still riding in the small-bore class. He's also still the only rider ever to do it.