Push back and automate
Only the rinky dinkiest of operations I've worked at have used names, initials or any permutation thereof for logins. Everywhere else it's always been based on some kind of unique identifier, be it staff code or employee ID or whatever you choose to call it.
I'd suggest that the author push back and recommend that mechanism instead, as it rules out the possibility of duplicates. If staff codes are not in place yet, make it HR's problem to cook up such a scheme - that should put the project on the back burner for at least a month.
Also, those 20 hours could certainly be spent more productively developing scripts to make the relevant changes - this might also lead to the development of tools to significantly automate user management workflow in future.