Can I make iCloud Preferences forget an old AppleID (mac OS SIerra)?
A quick check shows that quitting System Preferences and deleting the folder...
~/Library/Accounts/
...followed by subsequent log out and log in, gets rid of the pre-filled Apple ID.
After that a new sign-in request from iMessage/FaceTime appears.

Other than that there seems to be an abundance of account types listed in the database within the "Accounts" folder, so logging into those might be required again.
To see a list of account types held in the database, run this command in Terminal (to be found in /Applications/Utilities, or via Spotlight):

echo 'SELECT ZACCOUNTTYPEDESCRIPTION FROM ZACCOUNTTYPE;' | sqlite3 ~/Library/Accounts/Accounts4.sqlite


...and press enter

This has not been tested for any long term side effects.