Inconsistent behavior when replacing file with different format

[psychoadept]

I had a missing track which was in .mp3 format. What happened was this:
* I hit Locate...
* I found the track, but in .m4a format. MusicBee accepted the substitution
* I edited Original Album and Original Year in the main panel
* The change appeared in the vertical tag editor
* I edited Original Artist in the vertical tag editor
* When I hit save, I got the message shown: "the following files no longer exist at their expected location", and says it's still looking for the .mp3