Your change is based upon the original tickets rules so it should be the $100 or whatever it was then (they can look it up)

I believe but don't guarantee the answer to the second is you're fine to use for your xmas trip, but wouldn't be able to take credit from that flight and extend it again. I'll be interested to see whether others agree or I'm incorrect here.

ps you're better off just applying it against your upcoming europe trip and being done wth the credit then booking whatever you need however you want