I didn't know they ever added non expiring passes to room keys (not dating they didn't - we never had ours connected, the passes were always separate).
I would think if your old room key reads parks days that you can take them to guest services at a park and have the keys converted to tickets, then combine those tickets into one ticket, and that ticket would probably be able to be added to your magic band.
I don't know if you'll be able to do anything before visiting guest services though.