I think you have to write some extra code. For example:
if 'MICHAEL89' in map(lambda name: name.upper(), USERNAMES):
...
In this case we are forming a new list with all entries in USERNAMES
converted to upper case and then comparing against this new list.
Update
As @viraptor says, it is even better to use a generator instead of map
. See @Nathon's answer.