Rational
When configuring an account we can define if party is required and then the system force to define party in all moves of this account.
But actually it doesn’t get care about changing this configuration, so we can get movements in an account with and without party.
Proposal
Add a validation in same way the stock module do with product Type and Default uom fields. So if an account has moves, the user won’t be able to modify party_required field.