I am thinking it just a costume and not some sort of retcon. I just don't see them going the retcon route even if they might want to. It would take too much away from Scott to undo some of his more radical days. The X-men will finally feel whole with Scott, Logan, and Jean all back as members (and Xavier too if he ever rejoins). I am excited to see how this all plays out and the effects it has on other characters (namely Emma and Jean because eventually Scott will have to deal with them).