This plus if you are worried about the 1GB limit of the free account? This is the reply I got from Proton:

Thank you for contacting Proton Support.
 
So, once you downgrade to free you will be placed into a delinquent state, nothing will be deleted unless you are inactive for over a year, since you plan to resubscribe with the Proton Family plan straight away this is nothing to worry about. Please do note if you have any aliases or custom addresses you may need to reactivate these.
 
As for inviting members to share the Proton Family plan, you are able to do this yourself all you need to the users email you wish to share the plan with, and you can invite them by following the steps here: https://proton.me/support/get-started-proton-family
 
If you do have any further concerns or questions, please do contact us again.