When you upsample (increase bitrate) the quality will never improve. You would have to re-master the MP3 from the source, whatever it was.
It would be a better idea to downsample all of the tracks to the target bitrate to save processing by the DSP, but that's really not necessary either. From a "technical" standpoint, the ideal source bitrate would equal the stream bitrate. Some people also believe that even multiples help downsample quality as well -- 64kbps being half 128kbps, 32kbps = one quarter 128kbps, etc...
In my opinion, were you to have all 320kbps MP3s that you were forcing your DSP to downsample to 64 and 24 kbps, you would probably find poor quality because the difference is significant enough that you may introduce artifacts (=noise) and you will "lose" a lot more quality in the conversion that if you had 128 kbps MP3s. To put that another way, to your ear, the stream would "sound" far worse than the source.
Also, keep in mind that 64kbps stereo is actually 32kbps per channel, so they "sound" essentially the same. Stereo depth however, is sometimes more desireable than frequency response.