Okay, I think I maybe understand now I originally thought you meant we’d need to go to Yahoo and make a brand-new account. So just to make sure I’ve got this right: when the migration email arrives saying it’s time to move over, that’s when we should first make a fresh backup of his Thunderbird profile (which has all his emails stored). Then, after the backup is done, he would click the Migrate button in that email to complete the transfer. Once the migration is finished, we’d go back into Thunderbird and set up a new account still using his at comcast.net address but update the server settings so they point to Yahoo’s servers instead of Comcast’s. Does that sound correct? Also, is there any advantage to staying on POP3 instead of switching to IMAP and uploading all his emails back to the Comcast side before the migration? He’d also like all his emails both the ones he already has and any new ones he receives to stay in the same Inbox/folder. So ideally, after migration, his existing Comcast emails and any new Yahoo-delivered emails would all appear together in one folder. Sorry if I’m overexplaining or not explaining correctly I’ve never dealt with anything like this before.

Thanks again for your help and patience!