You could only change them from 128 to 192 be redoing them.
If you have a file of a certain bitrate, you CAN make it a higher one, but obviously it wont sound any different as the frequencies that would make the difference from 128 to 192 have already been removed.
The best thing to do is to find tune with a fair bit of bass and do one of each, then you can hear the difference for yourself and decide if you want to fit more on a cd (smaller file size for 128) or want to hear bass as deep as it was meant to be!