Just thought I would report back as I was questioning myself about the old bands.
We have bands from our last trip from 2014 and decided to try these out instead of purchasing new ones. We had zero issues in the park....worked for our room, lighting lanes linked fine and even though we were warned about photos, we were able to link them (most had a spot for you to link your band when you found your picture at the ride exit - but it also automatically linked on things like Pirates, Haunted Mansion, Buzz Light , Frozen and even the video on 7DMT). The only place we had an issue at was Disney Springs but they are apparently 3rd party so aren't connected the same. We did pick up the cards just in case (so maybe this is what did the auto-link?) but just reporting back in case anyone else is questioning.
Also, the resorts and stores did sell the new Magic bands...so no issue purchasing on site.