I started ripping before FLAC was a thing, and my first albums are all in 256k CBR mp3. I've never seen a big reason to re-do them. Yes, the files have gone through LAME's acoustic filter so they don't match the original bit-for-bit, but I mostly can't tell the difference. There have been a few times when I got annoyed by audio quality issues, but it's been 50/50 that it turned out those were also on the cd, so either a deliberate choice or a mistake during mastering.

As others say, there is an issue with compounding losses when you transcode mp3 to vorbis or opus, but I've never needed to do that -- and when I export, it's to move the files to lower-quality devices anyway.