Huh. I was apparently assuming.
I use Mp3tag to copy from one tag to another in the same file fairly often using Actions (format value, see
http://help.mp3tag.de/options_format.html). However, when I tried it specifically with TYER, it didn't work. I added an editable field for TYER in the left panel, and any text value I entered went into TXXX/TYER. It would seem that we don't have high enough clearance to fiddle with that specific tag value. Perhaps it's a threat to national security. You can try defining the value of your custom tag as %year% instead and it might give you the results you want, it might not.
As far as I can tell, any software that can edit tags will always define TYER as a four-digit number which will then be referred to as "year" in the interface. I'm starting to get a bit curious regarding what is in your TYER tags that can't be displayed in the year column. At any rate, I thank you for the opportunity to learn something new about ID3 tags and how inflexible they really are.
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