There are some exceptions, but on most US-EU tickets it doesn't matter much--the ticket will reprice using historical fares (and the advanced reservation requirement measured from the original ticketing date) as long as you don't change the first flight on the ticket. If anything, you risk some of the cheaper fare classes drying up if you wait to make the change (although, on the other hand, they could also open up later too). If you already know what the change is that you need to make, I'd just go ahead and price it out to see what comes back. If you don't like what you see you can wait until after departure to see if things get better.