PowerDeleteSuite might work for that, I've been using it for awhile to edit all and delete comments. I'll try and see if I can edit all without deleting....

EDIT: Yes you can edit all comments without deleting.

Here's the link to it: https://github.com/j0be/PowerDeleteSuite

EDIT again: There's a new fork of that project that adds a 5s delay to help it work better. The original is now hampered by reddit's bot detection stuff.

newer fork: https://github.com/pkolyvas/PowerDeleteSuite/