Unfortunately, as you noticed, the system was updated and a new requirement was added. Your original UID was grandfathered in (allowed to continue as it was) until you changed something on the page where it is, then you had to update it. Once it has been changed, there's no way to revert back to it.
The same thing happened with many clans that had certain special characters in their names or tags.
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I see. Well, thank you for the response! Time to think of a new ID then. ;)