How to change a Mac/iCloud account the right way
You can’t split an AppleId so either one party keeps it or the other keeps it. Then you remove the account from all devices not belonging to the new owner. 

When you delete an account, you can choose to retain the data locally or erase it. Without a backup, this choice is permanent. 

Since app purchases cannot be transferred

Data like Safari history(other than copying from ~/Library folder) is more amenable to an export or migration once you detach from iCloud keeping in mind only one owner of the account remains. To deranged things, one option is to change the email address and all other info of the account so it matches one identity. This preserves all the devices' sign-in and data.

As comments say, both old and new account are not Apple domains, Logging into AppleID account page and changing the email address would work. Verification is required ofc.


  You can change your Apple ID to another third-party email address if your current Apple ID is a third-party email address. You can also change your Apple ID to a login alias that already exists. 


https://support.apple.com/en-us/HT202667