I sometimes find that I cannot update certain tags within mp3tag, for example renumbering tracks in the format "nn/nn", when they only have a single digit track number.
I figured out that the files have older tag versions. I also figured out that I can remove tags and enter them again, which will then save in ID3v2.3 format.
I don't quite understand the Mp3Tag options though. Eg. for Mpeg, I have the options set to read all tags, write only ID3v2.3, and remove all tags. That suggest to me that when I load older tags and re-save the tags, it would only write ID3v2.3 tags back.
I found that I can select all tracks and then select "Remove tags" (Ctrl-R). I can then Ctrl-Z (undo), which restores the tag text that had been previously loaded. This seems to change the tags to the desired tag format, as I can then do the edits as expected.
Is this correct behaviour? Perhaps it would be clearer to have an "Upgrade tags" option, that converts tags from older formats to the chosen write tag format.