Sign out your primary Apple ID from the Mac system preferences an option comes when Apple ID is completely signed out , click on create a new Apple ID you can choose the mail address as example1@icloud.com
Note : before signing out , make sure your primary Apple ID accounts credentials should be known like apple id password , three answers for respective questions , date of birth , rescue mail address and its password .
The new Apple ID will be replaced with previous Apple ID and will onwards act as a primary Apple ID .
See a thread for old version of Mac sign out from secondary authentication ac… - Apple Community
In the screen shot of it you can see option to create new Apple ID , just click over it the upcoming steps will slightly vary for type of account that is again to be signed in , but the concept will be same .