Unless they're getting a paper refund check it's not a big deal. File an 8822 Change of address form (and applicable state form) to get the address updated. If a paper refund check, I'm guessing you may have to wait for it to bounce back as undelivered. No clue whether calling the IRS will work. Anybody else have an answer?
Ive found the 8822s get processed pretty quickly, I had to do one for my daughter last year because I screwed up, and her IRS account showed the updated address in a couple of weeks.
