How to I connect my registration form that is already set up to a different account?

If you mean when a new submission is received to your form, and you want that to be sent to another form's submission page too, unfortunately, this isn't possible. Especially, if the other account is owned by a different user.

Now, if you own the other account that you're referring to, we can transfer that to your new account instead.

Just log in to that account and post a request here. Mention in the request the form link along with the username/email of the account where you want it to be moved.