The problem is that, like you mentionned, you cannot change your user's UID when it is logged in a session. You have to use another user account to proceed.
But you don't have to create a new user account, promote it to admin, log out, log in to the new amdin account, change your primary account's UID, log out, log in to your primary account then delete the new admin user just change your UID. ;)
You can boot into recovery mode (it's an option that appears when you start up your computer). This will log you in a root session. Being logged in root -- and not your usual user account --, you will be able to modify your UID.
Because the recovery mode only works in command line interface, once logged into a root session, you will have to use Luis Alvarado's command:
usermod -u NEW_UID your_username. Follow ddeimeke's instructions to update file permissions. Then, reboot your computer (
reboot), so you can boot in normal mode.