It took some additional fiddling around to get POP email account to work, but adding the new account and using a bogus email address got me through the first level of the puzzle. It looks like some email may have lost in the process, but I didn't have anything archival on this machine. If you try doing this for a machine that has vital archival email, you may need to use Time Machine to recover the older stuff. Because I used my Apple ID to set the account up on my laptop (which I upgraded to Big Sur), and as a result a duplicate copy of the account appearing on my desktop.
Thanks