flac 0 through flac 8 (I think that is the highest) are lossless, compressed formats. dBpoweramp also provides Uncompressed flac, which is not compressed at all. It is basically a wav file with a flac header. If anyone is concerned about flac compression, uncompressed flac is a good option since it has the metadata capability that wav lacks. There are versions of wav with metadata, but they seem to be pretty nonstandard. I think most players play uncompressed flac.
If the m4a files are lossless, then I would suggest converting a couple to uncompressed flac, flac 5, wav or one of the Apple formats and comparing them to see if you hear a difference. Then re-rip the same album to your preferred format and see if you hear a difference. It is easier to do the experiment than to re-rip everything. You can also re-rip at a later time.
If your m4a are lossy, then definitely consider re-ripping.
Obviously, take your time upfront, before launching a time consuming process.