Well it was a bit of a scary proposition, but I converted my library from ALAC to FLAC, since it appears that FLAC files have internal validation that ALAC is lacking.
Using dBpoweramp it only took a couple of hours for the batch conversion, and it went relatively smoothly.
The latest version of dBpoweramp moved to 64-bit, which has made conversion noticeably faster, though I have experienced a few issues with their ALAC support. (which has been reported to them)
Both multichannel files (fixed) and mono files (pending) were reported as corrupt by the tool.
For those files, I just used Media Center to do the conversion instead.
I could probably have used the previous 32-bit version of dBpoweramp for the conversion, but didn't want to have multiple versions of it installed at once.
When Media Center performed the conversion, the mono files were changed to being stereo though (with no apparent increase in file size) and the embedded PNG artwork was converted to JPEG.
I no longer seem to have a copy of the original PNG art backed up anywhere, and I'm not sure how to extract it from the ALAC files.
Neither is a big problem, as it was less than 20 tracks, but it is annoying.
Before doing the conversion, I disabled auto-import in Media Center (I was converting in place and leaving the originals) and once it was finished I did a find & replace on the filenames to update them from
.m4a to
.flacI also had to update the file type from M4A to FLAC too.
This meant that all my tags were retained, as some of them are only stored in the database rather than written to the file. (I'm not sure if I should just set everything to write to the file?)
Running the free version of PerfectTUNES seems to think that the audio data is the same for all the converted files, and I have just finished running Media Center's analysis on them without any errors, so the conversion seems to have been successful.
Overall, moving from ALAC to Level 8 FLAC (highest compression) saved me about 4GB from the ~200GB of files that I converted - not that I'm doing this to free up disk space.
I still have a backup which contains my Media Center library and all the original ALAC files, so that I can always revert to that if necessary - but hopefully I'll never have to.