How to change a Mac/iCloud account the right way I'm helping someone who is divorced and needs credentials changed on her iMac. It is using a local account with her ex-husband's name and iCloud account. I would appreciate some assistance with a strategy so I don't overlook anything. 

Access point SSID and password have been changed, as well as the router's password (which was still set at factory default!). Her email password has been changed, along with website passwords.

I have created a new account on the Mac with Administrator access and changed the password on the old account. Verified it has no remote access apps. Ready to create a new Apple iCloud account based on her own e-mail address.

The things that are on my mind due to knowing MacOS less than Windows:


What happens to apps purchased on the old Apple ID once this machine is updated to the new one? Must they be repurchased?
iCloud's list of devices includes this Mac, her kids' iPads, and her iPhone. But it also includes his iPhone. Best way to move these devices to the new iCloud account without disturbing his?
Best way to copy Safari and Firefox history/bookmarks/etc to the new account?
Any other important steps?
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