We had these on our last trip in September and though we entered them in and the system "saw" them -- as in they showed up as attached -- they wouldn't let us book FP+ with the days from the old non-expire tickets, which was a PITA. We ended up at guest services on day one for about 75 minutes while they added the tickets to the magic bands and copied over the FastPass+ selections from my magic band (since I had an AP) to my family's. Luckily, it was low season and they were able to do it for all but 7 Dwarves Mine Train without too many issues. (At this point I also had a lot of documentation from all the people I'd talked to pre-trip about this, and lots of notes on the account, apparently.)
Well, at least that's what we were told ... Frequently either my son or husband's supposed FastPass selection wouldn't register when we touched the Mickey reader, but they always waved us through when I showed them the schedule on my phone. And my son had lots of problems with his band for admission, culminating in our last day, when we didn't even try to go to the park until after 5 ... Only to discover that they'd screwed up the number of days on my son's ticket.
Instead of fixing the old ticket numbers, or whatever, they had just issued new ones for the days left, shorting my son two days he was supposed to have. We ended up back at guest relations for another 30 minutes and they, eventually, just let us into the park for that one evening and we gave up and threw away the last day he should have still had left on the non-expire ticket because it wasn't worth the fight and hassle.
All that being said, I hope you have much better luck. My advice is talk to whomever you can via phone and email before your trip and document, document, document! I should have had written information from the first guest relations cast member on day one but after the long wait my outside the park gates my son was bursting at the seams to just go in already -- and who could blame him? I had the info in my phone on the app that looked right so I didn't bother getting anything in writing about number of days, etc. in case we had later issues which, sadly, we frequently did.