I’ve tested and made an Action that should do it.
djay Pro.mta (354 Bytes)
Before:
After:
It works with tags formatted as either YYYY-MM-DD or DD-MM-YYYY and replaces any separator to /. It uses RELEASETIME if already there, otherwise uses YEAR.
As @ohrenkino points out, RELEASETIME is an official tag similar to YEAR or DATE, as opposed to RELEASEDATE. You can open the Action and replace any mentions of it to whatever is needed as I couldn’t find a list of Djay Pro’s supported tags. .FLAC and VorbisComment’s supported tags could also have an effect.
Refer to this to add the Action:
Merge it add actions from another database - Support - Mp3tag Community