Is user 'john' in the new server your own account? And is it set to be an admin level?
Another way to avoid the conflict is to change login/email address of current user 'john' in new server.
Attempt to restore the course again - shouldn't find 'john' so no conflicts. However, you did need to check the new 'john' account to see if it's Admin level and adjust appropriately.
If this one course has the issue, how many other course backups do you have that will probably have the same issue?
Think it's always been recommended NOT to assign an admin level user/account to a course as teacher for this very issue.
Am aslo curious as to why upgrading went to 4.2.3 when the highest/most secure of that version is 4.2.11
https://moodledev.io/general/releases/4.2
'SoS', Ken