I just wanted to report that all went well (apart from all the crashes and MC thinking files with different filenames are duplicates and will not convert them). But it took a while. The album gain tags in the flac files, which I had imported to custom MC fields, were not converted so I had to also use an external tool to re-apply gain, which is a time consuming task when you have a lot of files. I wish MC would as an option support converting the actual album gain tags in the existing files (even though MC does not use them), at least if the user has imported them into custom fields (why exclude just those fields from conversion?). MC also created a playlist folder in the destination which I did not want and I could not find an option to prevent this. Furthermore, MC created a hidden Database.mpl file in the destination, which I do not know the purpose of (at least it does not seem to speed up resync which, as MrC pointed out, is not particularly fast).
But I am happy. Applying the necessary tagging script and gain tags to the mp3 files with external tools did not trigger a re-sync (so it seems to be not an actual - two way - sync operation, which suits me fine). A change of the tags in the flac files, however, did trigger a re-sync of just those files, which was exactly what I wanted. I just have to remember to apply (album) gain to the whole album if only some files in an album are changed and to apply the necessary tagging script to just the changed files (the script does not work for the unchanged files which have already been through the script). The cover art converted nicely (embedded in each mp3 file and a folder.jpg on top of that in each album folder) as well as all custom MC fields _except_ the album gains tags I had imported (strange). It worked using expressions for the file path.
Thanks for the input!
(I was curious, though, why Alex prefer to use two different versions of MC. I did keep a copy of MC 14 when JRiver said that MC 15 was in beta but after that I think I uninstalled MC 14 since I could not see any difference in the features for audio files that I use for tagging, organising and searching my music; I do not use MC for ripping or playback or as a music server or the cover art or theatre views, or for anything but audio files. After the introduction of MC 15 final, improvements to the features I use have come gradually - locked views (great!) and some improvement in the speed of the tag editing action window (but I would need more when having a lot of files displayed at once), and Matt has a 40% speed up of a complicated view waiting for me in the next MC build..., and you, Alex, said that someone at JRiver is actually working on improving the audio file tagging support (promising!).)