Our user registration settings are configured to allow usernames of 3-20 characters (the maximum XF supports is 50). However, when I try to change it for you, it just throws an error, "Please enter a value using 15 characters or fewer." (15 is the XF default character limit). Not sure what's causing the inconsistency between our configuration and the actual allowed limit when we change usernames, but once it's ironed out, we should be able to get that changed for ya!
WELP, "Jack" was already taken, and even "Jack S." gave me the same "15 characters or fewer message" soooooo it looks like it's something that the other two'll need to look into to see where it's busted in configuration. I've tried toggling and re-saving the Registration settings again just to check and make sure it wasn't some sort of initial caching problem with migration or whatever, so... it's up ta their magic now!