Quick question - if you have 20 minutes on a certain O/D in practice, and do NOT run that O/D in games, do your players' IQ progress slower than if you both practice the O/D and run it in games?

Switching the defense I run and have been getting some painful results...is it worth running my old defense for a season or is there a benefit to running the new (lower IQ) defense in season 1 to progress it quicker?

IQ only increases with practice time. There is no added benefit from IQ progression by running a certain O/D in a game.

I've always ran the old offense/defense for the majority of the transition season.

it really doesn't make sense to have a team run the new sets, when every player on the team is a D IQ. While having seniors that are A/B IQ in a set that is not getting used. It also does not "speed up the process" by playing the new set.

the earliest I'd play the new set might be CT time. BUT you said you're only using 20 practice minutes. So i wouldn't even switch to the new set until next season. I recommend at least bumping up to 25 min in each set, during a transition season. And again, that's "at least". I've always used more than 25 in this situation. But to each his own.