So, we are a small church and we have a Mac for our office admin. The Mac was set up with an AppleID by someone who has long left the church and we're not even sure who did it. We don't recognize the partial email or know the answers to the security questions.
How do we recover the existing AppleID or set up a new one for this Mac w/o losing access to the existing documents on iCloud?
You probably can't unless you can find out the account log-in information, particularly if the data are in iCloud only. Apple's security around an Apple ID is pretty strong and they won't open it up unless you can prove positive the account is yours with account recovery, or you are the inheritor of a deceased person's things. I could post links to articles about those but since they don't apply there's no point in my chasing them down.
If Apple ID is set say example1@gmail.com , knowing Apple ID and password is not sufficient , you need correct answers for three security questions , if you can't sign in with appleid.apple.com convert the account into 2FA Two-factor authentication for Apple ID - Apple Support
Get a verification code and sign in with two-factor authentication - Apple Support
If four Macs were signed in with same Apple ID and password and same credentials convert the secondary authentication account into 2FA for individual Mac ( separately ) .
If you don't know admin password for Macs they can be reset from recovery mode individually If you have forgotten admin password comp… - Apple Community
Note : mention which OS and version is used in Macs .
If you can access the Mac the Apple ID can be read directly from Mac If you forgot your Apple ID - Apple Support
The documents and files and other data will never be lost when iCloud Drive and desktops and documents were always turned on Use iCloud Drive to store documents on your Mac and iOS devices - Apple Support
Add your Desktop and Documents files to iCloud Drive - Apple Support
Employee Left and we don't have AppleID login, password, or recovery info
