I’m just brainstorming.
Will they forward mail-in ballots? Some types of mail can’t be forwarded. If people have temporary addresses maybe they can just get the mail forwarded. It can take two weeks for the mail to start showing up in the new location. There is still time.
I think I read in Maryland You can print a ballot yourself. That’s how it should be everywhere. Maybe the effected states could allow that considering the circumstance. I don’t know how complicated that would be.
Regarding what jellies said above, I know my mail person will hold mail for weeks for me even if I haven’t put a formal hold in my mail. Technically she is supposed to bring it back to the post office if I haven’t picked it up, but she just keeps it for us in the mail station where I collect my mail. My point is, I do think the post office will take care of the mail and deliver it if at all possible to wherever the person is. This is what the USPS is known for and why most of America loves our USPS.
Another option is delaying the election in those states. The electors don’t vote until mid December. Many states accept mail-in ballots up to two weeks past Election Day anyway.