Copying List Elements To Another List And Changing Them
Your second version is better than your first version, for two reasons:


All else being equal, building a new list is IMO easier to reason about than changing a list you're iterating over.
You're iterating using for...in rather than iterating by index using a range (and the range in your first example is hardcoded rather than being over the len of the list, which is doubly bad).


The most efficient/pythonic way is to use the comprehension syntax:

currentUsersLower = [user.lower() for user in currentUsers]


(Also, standard Python style would be to name these current_users and current_users_lower!)