iCloud Mail

This issue is occurring from which device iPhone / Mac / iPad what are the versions .

Your preexisting mail address is used with which mail services , yahoo , google ...etc do these account exists on server or you had permanently erased from their server .

We don't need your actual mail address ( as its never to be posted here ) , but need an information from where are you trying to sign the mail address Mac or iCloud.com ? and what upcoming error do you face .

From www. iCloud.com click on account settings - in manage section the Apple ID can be switched , click on mange , enter admin password , in account section click on edit , click on change Apple ID , enter the preexisting Apple ID @iCloud.com and click on continue , in the last click on done to save data .

Read the article https://support.apple.com/en-us/HT202667

The preexisting mail address is a normal web mail address say example1@gmail.com or created with some mail services like yahoo , hotmail ....etc it was never created or treated as an Apple ID address .

The Apple ID that is signed in Mac system preferences could be example2@icloud.com

Suppose you had created an Apple ID say example1@gmail.com in the initial stage of creating the account and after that signed out it from system preferences and then signed in with Apple ID example2@icloud.com .

In that scenarios if you click on create a free new Apple ID in www.icloud.com then it will create a new Apple ID address .

the step you are following is trying to sign in within iCloud.com is a very normal mail address which was never created as an apple id , but used with iCloud mail accounts .

This web mail address when typed in the link https://iforgot.apple.com/appleid#!&section=appleid

Will not a shown a green check mark and show Apple ID not verified , if the Apple ID address was correct and it exists on icloud server will always shows as Apple ID verified .

However , the normal web mail address that you are trying to sign in within iTunes will also not work .