The user.php file in the ModCP folder isn't set to take modifications to the user fields with the exception of pics, avatars, signatures and reputation (may be one or two others as well).
If you are going to add things that the mods can change (like birthdate, homepage, etc...), you will need to add the logic to the user.php file to update the database.
Without that, 'save changes' won't do anything.

:
If you want to see an example of how this would need to look, I have a hack that allows mods to change usergroups. Look at the changegroup.php file in the zip -- that will give you an idea of what all you need to add so that the interface and database pieces are both there.
Here is that thread:
https://vborg.vbsupport.ru/showthread.php?t=62018