Re: "forgetting" people permanently

@ashbymcdonald  You could install NRaas RelationshipPanel and ctrl-click on a given portrait to remove the sim from your sim's relationship panel.  This would work before or after the move.

Or, if you'd like your select list of sims to all move to the new town, you could use NRaas Porter to pack the sims you want and then place them in a new save.  It's like placing a saved household from the library, except you can pack a larger number of sims, and when you unpack them in the new save, they'll go back to being in their own households.