The main thing to be careful about is not to leave some subscriptions such as iCloud storage and Apple Music without continuous coverage or else you lose content. I have never done this so I don't know how strict they are in terms of timing and you may want to ask Apple about that. So if a person is part of Family 'A' Apple Music subscription and wants to be part of Family 'B'. If they leave A and then join B there will be a short interval when they do not have any subscription and there is the risk that their playlists, etc. will disappear because deep down in the legal documents: https://www.apple.com/legal/internet-services/itunes/us/terms.html you will find, "When your Paid Subscription to any Service ends, you will lose access to any functionality or Content of that Service that requires a Paid Subscription."
Strictly speaking if it takes 1 minute to jump from Family A to B one could be at risk of losing content and functionality. Again, I don't know if Apple is that strict but that's their Terms and Conditions adnd it is up to you if you want to risk it. Then the solution is for each user to take out an individual subscription to "bridge" them over the transition, but that's an annoyance and extra expense.
The best thing to do is ask Apple:
Contact Apple for support and service --> "See a list of Apple phone numbers around the world." Click here --> https://support.apple.com/HT201232
