To merge two separate accounts into one, you'll first need to cancel one account then add that person as a new member to the existing account they would like to be combined onto (see screenshot):
Because the patient has already paid your Lifetime Activation fee, when you sign the patient up for the new account, change the Lifetime Activation fee to what they owe that month, or waive it if they have already been charged for that month. You can find a detailed article on how to waive the Lifetime Activation fee HERE.