well, it takes just as long as freshman take in the old set, to grow. not an exact answer there, i mean, you basically feel most of the pain in the first 2 seasons. by the third season you have junior year IQ in your seniors instead of senior IQ and thats the only downside. so the pain is really over after 2, although if you are trying to win a title or something, that lack of senior IQ can definitely take you down a notch.
i suggest you practice the new set and play the old. the improvement from playing something is VERY slight. i usually play the old set at least one whole season. the next season it depends, if you have a lot seniors i might still play the old set at first. juniors will have 1 season in each set starting the season so its a wash there to start and by end of season you have 2 seasons in the new set, to 1 in the old set, way better to play the new set (for juniors). for seniors you start with 2 in the old, 1 in the new, so only if you have a senior dominated team would i even consider starting with the old set. by the end of the year its a wash for them so you ALWAYS are switched by the end of the 2nd season. generally speaking i just switch for the second season and am done with it. if you have a very young team, you might want to switch earlier. how well suited your team is to the new set vs old can also be a factor, but really, if you just switch what you play after 1 season you are doing it well enough.
pump up practice time? not much. don't exceed 25, you can go from 20 to 25 if you like.
if it worth changing? cant answer that. depends what you want to do, what your goals are. if you want to move up ASAP to the next level, no, its not worth it. if you are trying to build a dynasty, its worth playing what you want to play so you enjoy that long time there more, and can build for what you want to build for. that is up to you.