I know how hectic it can be moving; have done it myself several times over the years with being a military spouse.
No worries on the postage and it's totally up to you how you're going to handle the return postage. As mentioned in my PM back to you, I'm still including the stamps since I'm HOPING that all I have to do to the package is add a new address label instead of using a new envelope. Thinking I might just use a big piece of cardstock to cover the entire envelope since it probably has all the postage stickers and stuff on it.
I don't feel that you should cover the return postage since I'm wondering if I didn't pay attention and you did indeed change the due date BEFORE I shipped. As soon as I get the package back and re-mail, I'll either send you the new DC # via PM or just post it here.