Changed User ID Whilst Logged In (Still Logged in Now) I was changing some settings when I came across the user id in the accounts section. For some inexplicable reason I thought it meant my network User ID, ie what I would identify to a router with. No idea why I thought this. Anyway.

I changed my User ID.
I am still logged in.
I cannot open any programs, including terminal, to try and change it back.

Currently open  programs include: Finder (downloads folder), Firefox, Sublime and Outlook. 

Currently I'm afraid to reboot. However I do have access to several other Windows/Linux machines and lots of flash drives if I need to download/install/prep a live boot/etc.

I need to figure out how to change my User ID back.

Solution I'm currently trying to work:
I have a file explorer open. I have sublime open. 

If I can find where OS X stores the User ID, I might be able to navigate to it and change it back to whatever it's supposed to be (501 I think?).

If anyone else has any ideas or solutions, I am all ears. In any case, I will post an update when it's solved one way or the other.

Edit: Update (since I said I would). The problem has been solved, Klanomath's solution below worked perfectly first time through. Thank you!
Reboot your Mac to Single User Mode (cmdS while booting) and enter at the prompt:

/sbin/fsck -fy
/sbin/mount -uw /


Launch opendirectoryd:

launchctl load /System/Library/LaunchDaemons/com.apple.opendirectoryd.plist


You will get some error messages - repeatedly.

Then enter (replacing username with your real user name):

dscl . -read /Users/username UniqueID


You should get something like 272 (if you changed it to 272 accidentally)

Now change it back to 501 again and check the result:

dscl . -change /Users/username UniqueID 272 501
dscl . -read /Users/username UniqueID


which should yield the original 501.

Then enter exit and boot to your GUI or better: reboot your Mac immediately with shutdown -r now to avoid errors based on the premature launch of opendirectoryd. I got some kextunload errors trying to shut down/reboot after booting directly to the GUI.