I'd more liken it to subsequent releases of a fighting game.
You work to level up your character in one game, and fight lots, refining your strategy, laring patterns, etc. Now SF2 Turbo Edition has released, and in the transition, hitboxes are tweaked, attacks do different percentages of damage, timings are different. You can carry over most of your skills (your previously trained Pokemon) from the first game, but you now have to work to make sure those skills fit the new game, as different tactics are changed.
Also, you don't really have to relevel due to the existence of EV respeccing methods - plus it's being made easier all the time to train.