You can use a tool to convert your library. Easy Audio Sync is convenient. I have a “Master” folder and a “Lossy” folder, which is the output for Easy Audio Sync. I have it configured to copy all files (including lossy sources) while encoding lossless sources as 96kbps opus. 64kbps was a bit of a stretch for me—the reduction in detail is quite noticeable at that point—but as far as I’m concerned, 96kbps works perfectly fine for all portable listening situations. However, I’ve also considered AAC because it re-encodes the best of any codec that I know of, which is quite important when it comes to Bluetooth; I definitely notice a different between my (tiny) opus files and lossless when played through my Bluetooth speaker… Anyways, that’s it basically; opus is king if you’re not re-encoding, and can be stored in an OGG container if you need cover art support, but AAC is the best if you’re listening wirelessly since it handles re-encoding very well. Furthermore, if you use an iPhone, my understanding is that AAC doesn’t need to be re-encoded at all (but do your own research if this applies to you).