As computer disk space is significantly cheaper than 10-20 years ago, best practice is to rip CDs as lossless (alac/flac) to have a bit-perfect copy for archival purposes. (imho,
XLD is the best Mac choice in regard to error correction, which is important for older CDs)
I do prefer alac (Apple Lossless Audio Codec) due to the perfect integration in iTunes/Music.
300 CDs populate ~100 GB of space.
I am not claiming I could hear a difference between 256 kbps AAC and ALAC on my Mac with my current speakers but I like to have a lossless master and convert from that depending on the device / use case.
This way only one single lossy conversion is performed, independent of the target codec / bitrate.
A.e. for my iPods / iPhone I have ticked "Convert higher bitrate songs to
192
or
256
kbps AAC" depending on the device. For my headphones / car stereo that's more than enough and saves space.
So I have to concur with
@clueless88, re-ripping existing CDs is not mandatory if you already have a 256 kbps AAC rip - although that's exactly what I did to put my old Power Mac G5 to use. For your older AAC rips (I suspect 128 kbps) I'd just re-rip one CD in lossless and check if you can hear a difference with your setup to decide if it's worth the hassle.
And a disclaimer: alac is not the recommended archival format as it lacks an internal Audio MD5 checksum in contrast to flac. But I keep my drives connected to an UPS and backed up.
edit: and glad to hear you found a viable backup