Microburst got it absolutely right:
Being at the dispatch room: Planning minima for the alternate apply.
Once you are airborne and the minima fall below the planning minima for the alternate, you are still good to use it and fly to it if necessary - the actual landing minima apply at that stage.
But if you decide to change alternates in flight, than you have to apply the planning minima again, as you are doing an inflight re-planning.
This is what's in our manuals (EU carrier). Will try to find the reference in the EU-OPS manuals later, but am heading off to work now.