OP: I feel like maybe you don't understand the entire issue here. Do you have a Uniglide or Hyperglide Freehub? Some models are compatible with both. You probably don't need a new rear derailer. Also, the derailer doesn't determine which gears you "lose" or "have." That depends on what gears you leave on the cassette. The derailer (should) shift through all of them.
Originally Posted by
Ex Pres
I'd try losing the next to largest, that way you'll retain your bail-out.
+1
Having a bigger jump to your largest cog is pretty nice sometimes, and you don't lose much. I had a 12-23 cassette on my road bike. I replaced the 23 with a 28, so now I have a 12-21 + 28 acting as a granny gear. Trying to lose the smallest cog(s) can get tricky as you can sometimes run into lockring issues.