I don't know if this can be done with MusicBee's tag tools, but the Kid3 external tagger (
https://kid3.kde.org/) can retroactively change ID3v2.3 tags with UTF-16 text encoding to ISO-8859-1 (Extended ASCII) encoding "en masse." The steps for that would be:
1. In the Kid3 "Configure" options, under Tag 2/ID3v2 set text encoding to ISO-8859-1.
2. Open the files into Kid3 and select those with UTF-16 encoding that you wish to change
3. From the Kid3 Tools menu, choose "Apply Text Encoding"
4. Hit the "Save" button.
For the changes to be reflected in MusicBee you would need to rescan the files.
As I see Zak just posted, taking a look at the plugin he mentions also sounds like a good bet.