I got bored and got a new team. They run uptempo. I hate uptempo. So I'm practicing Flex. So do I continue to run Uptempo in games while I'm practicing Flex until my Flex IQ's reach my Uptempo IQ's? Or does that slow down learning the new offense?

I have heard teams run their old offense until the new one is respectable - maybe even for a whole season

Playing the set in game doesn’t affect IQ, so you may as well go with the set your team knows best. This is especially true if you haven’t recruited many players yet. The other thing to think about is that playing the set you intend to switch to will be attractive to some recruits who already know that set. But most recruits only start out at C or less, so that isn’t a super value if it’s hurting you in games.