if you make the full switch (0 minutes in old set, 20+ in new set, play new set), expect to be trash for 2 seasons. often at a new job, this is going to happen regardless, and there is not a terribly large difference between winning 5 games and 0.
for programs that have teams that are competent, particularly with my own teams that i am changing over (i tend to change off/def a lot at times), i have found a soft transition that really eases things. basically, you want to switch to something like 15m in the new set for a whole season, generally with 10m in the old set, but less or none if you really don't have freshman contributing - playing the old set the entire way. the 2nd season in, you switch to full practice in the new scheme. sometimes i'll switch to the new scheme at the same time, other times half way through the year. it depends how much young players are starting and stuff - but for me, that is usually a lot - so usually this is when i switch.
the first season under this scheme, you tend to take a pretty small hit. at least in the post season, mostly you are relying on older players, who barely notice the hit on their IQ. the second season is a substantial hit, but nowhere near as bad as switching with a team full of F iqs, and often i'll end up switching after non conf or so in that 2nd season where my whole team including freshman is at least C- and that really isn't so bad. by the third season, you basically start with B IQs for your juniors and seniors and maybe even B+, and you end with A- or so, and you really feel like the post season, its a fairly small hit. its more meaningful if you are trying to win a title that year, but its very tolerable otherwise.