I was just able to customize my magic band for our December 14th arrival. We were 59 days out from our trip as of yesterday and I was having a lot of trouble yesterday doing online check in on the
MDE site. After I would go through the entire process it would give me a screen with a picture of Goofy and said "Oops.... Something went wrong. Please try again later." I tried clicking on Manage Magic Bands and it was still showing the generic what a magic band page is and link them to your account page. I also tried the Disney reservation site and was able to do online check-in and get a confirmation, but still not able to configure the bands. I just tried to do online Check in on the MDE site again this morning and got a confirmation page. Went back to the main MDE page and now the Magic Band link says "Personalize" and Viola I'm able to customize our bands. I'm really glad I was able to configure them and it does look like it's somehow tied to online check in.