Once onboard, if pax want to switch seats with eachother, upgrading and downgrading in the process, I don't see the issue. (ie. Husband lets wife sit in F seat.) As long as the orig open seats stay open. As for FA's, I think it's in their best interest to stay out of it unless a rule is being broken or they are asked for help.
The issue is the DYKWIA pax who feel entitled. Like when I politely declined switching e+ bulkhead window seat with her E- middle seat and the lady threatened to complain to the FA.
No matter how rules change, a DYKWIA isn't going to change their personality. It is in their nature to believe rules dont apply to them.