I have to change the email address on my account since it is one I no longer use. The site does not allow me to do that. When I enter the new address it says, "Email address already used." Well, where is it already used? I do not see it there; otherwise I would designate it Primary and connect through it. Nothing here seems easy.
It sounds like you have two accounts, and one already uses the new address. You are going to have to contact customer support to resolve this issue. I believe they can merge the two accounts and set it to use the new address.
Click the following to contact Adobe customer support staff for help:
Be aware that because of the pandemic, wait times are far longer than usual due to reduced staff and higher demand. Please be patient with the people to whom you speak. Like you, they are feeling a great deal of stress. A little patience and understanding will go a long way to helping you in your particular situation.