I would think that you could do the switch most anytime. Will it be where you can add a super if needed, if you have an extractor then you could keep the amount of frames needed down. One thing about a long hive and the follower board ( I have a TBH) is my follower board fits pretty good but a few bees and SHB's will get on the back side. Good luck with your project. There's probably other things I haven't thought of also.
I think the idea is to not need to put supers on it, for that defeats the purpose of going long.