Is there a way in DA to make a existing user into a reseller without deleting the account and making a new one?

There is a tool can be found at DA's forum which allows your move a user from a reseller account to another reseller account. search their forum is always the fastest way to find answers.

i bet you can create a new reseller account for that user, and use the tool mentioned above to move his domain to the new reseller account.