You need to go into thr old ID and give new ID full read write access in Users and groups.
Then go to folder /Users and start copy and paste,
You will ned to delete as you go and empty trash in old ID
or
Copy the whole lot to external disk, delete it all, empty trash and copy back to new ID
The only snags are if you use OnMyMac mail folders as they ate hidden in user ~/library Mail folders.
IMAP mail can just be set up again but POP mail - I have no idea where that is stored.
Needless to say take at least 2 separate backups
New ID must be full Administrator too.
Keep the old ID for a while just in case,