I use the Responsive theme a lot, so I too was not keen to do a “big update” on a production site.
However, I also have an almost “out of the box” demo site, and have updated that to Responsive v2 and on the surface of it at least, can’t see any difference. I haven’t poked around under the covers yet.
So not sure what the reference is to Responsive 2 Migration (as seen in the link in v1.9.9.0) but might just be because the theme is now v2 (as opposed to Responsive II – or Responsive Mobile as they now call it).
Their demo page looks the same.
I can’t see any chatter about it on their support forum either, so I’ll ask the question and report back here as to what the big changes are.
I haven’t compared anything between the two versions yet, though if I get no response from their forum support, I’ll do some comparisons and see what might be changed.