No as both are lossy codecs you will always lose quality doing so. You can do it with ffmpeg.
parallel ffmpeg -i {} {.}.opus ::: *.m4a
You may want to tweak parameters for bitrate, etc. But this is not needed at all, as AAC had patents that are now expired.
If you get .flac, .wav or similar lossless music, you should encode that with opus. But lossy to lossy makes nearly no sense (apart from specific players not supporting them)
GNU Parallel works well for this kind of thing. A lot of audio stuff is single threaded, so unlike video transcoding running multiple conversions simultaneously is a useful thing to do. The command is simpler, too:
Unless the m4a files are ALAC-encoded, you shouldn't do it. m4a files tend to hold AAC-encoded audio, which is a lossy format. Opus is also a lossy format, so you'll only lose sound quality, basically. You should only convert lossless to lossy.