Check the settings for EMail based registration for Locked User Fields
By default on a system not interfaced with a SIS (student information system) or other similar system for authentication ALL the fields in a users profile are unlocked. If interfaced with some ID/SIS, the decision might have been made to lock them so users couldn't change ... which also affects your ability to change things like the email address.
The explanation next to the form says:
"You can lock user data fields. This is useful for sites where the user data is maintained by the administrators manually by editing user records or uploading using the 'Upload users' facility. If you are locking fields that are required by Moodle, make sure that you provide that data when creating user accounts or the accounts will be unusable.
Consider setting the lock mode to 'Unlocked if empty' to avoid this problem."
Also, since the users profile is populated by some other system, maybe the place to look is that other system. Does it allow Upper/Lower case mix in a users EMail address? Anyway to get it to do the conversion from upper to lower *before* it populates the profiles in Moodle?
While you might have to resort to CSV updating of users with Caps in EMail addresses, seems to me those backended systems that are interfaced should offer that as a 'feature' ... contact whomever makes the product or whomever is in charge of it and inquire.
That sort of stuff exist to make your job easier ... not have to work to work. Soooo .... ;)
'spirit of sharing', Ken