This is actually a topic I approached a good few years ago, on these very forums; but now I have a legitimate reason to learn the answer.
Lately I've been on a 90's kick, and I'm not really ashamed to say I enjoy a fair bit of "Cheese" from the past.
On that note, I have an MP3 copy of Eifel65's album. I tried to re-rip lossless, but the CD has degraded and spits errors like no tomorrow.
There is a very real problem; almost all the tracks (even on the original CD) have a high pitched whine at somewhere in the 12khz region; its 110% a production error.
It makes it unlistenable in any circumstance where a tweeter, or even a simple 2way speaker system exists.
I need to edit these tracks, throw a compressor over the 8khz to 16khz region, and hope it mellows the hell out.
Problem is, they're MP3. So they're already lossy.
I think they're 192kbps..... Which gives me quite a bit of wriggle room to 'upsample' the track when I export it again.
Using simple numbers to make a point;
Assuming 192kbps cuts 20 out of the 1000 possible 'subfrequencies' we can hear\feel.
Is it then safe to assume 256kbps would cut maybe 15 out of 1000 frequencies; and those "15" were probably ALREADY lost in the original 192kbps compression?
I know "re-compressing" to the same bitrate has a very real risk of losing more detail than was originally lost, because it tries to re-analyze the track and 'shrink it' but its already done, so its MORE agressive to do it's job.