I actually went through this with my dad a few months back. We knew his original skymiles number, but could not find his password. And the email associated with the account was an old family email we had not had access to in years. Delta was no help in assisting us to regain access to the account. Basically told us to figure out how to gain access to the old email or figure out the password, or pound sand. He had been flying the last few years by using his number while booking, so we knew a fair amount of miles had accrued and didn’t want to give up quite so soon. We opened IT requests but never heard back.

Luckily, 3 or 4 months later my mom randomly found a password scribbled on a piece of paper in her file cabinet and voila, it worked.

We then started the process of combining all the new accounts he had created over the last year or two (in order to access the free WiFi) into his original account. He now has just 1 account.

Sorry I can’t be of more help. Just a datapoint that Delta wasn’t especially helpful and we got lucky on our own.