I have 61SE's as my front L and R. I've twice passed up the opportunity to upgrade to the 61SE/R's at very reasonable prices (mostly due to the disapproval of a girlfriend who's no longer in the picture
). Unfortunately, I've never heard the SE/R's, so take what I'm offering here with a grain of salt. The second guy from whom I almost purchased a set of 61SE/R's used to be an RBH dealer, and he said he was so impressed with the SE/R's that he stopped selling the SE's altogether. Now, granted, he was trying to sell me his speakers, but his opinion was that the difference between the two lines was pretty substantial. According to him, EVERYTHING was better in the SE/R's: imaging, soundstage, depth, range, space highs, lows--you name it. There's a few reviews out there if you're into reading them.
Incidentally, I'm not so sure you can just upgrade the drivers/crossovers. I called RBH and spoke with Jared directly about this, as my SE's are piano gloss finish and the SE/R's I was considering were the standard black oak grain finish. My plan was to disassemble the speakers, swap out the drivers and crossovers, reassemble them, and my girlfriend would never know the difference! Unfortunately, Jared said the ScanSpeak tweeter used in the SE/R's has a slightly larger overall diameter than the Vifa V27's used in the SE line. He said it would take some awfully careful routing or sanding to make the ScanSpeak drivers fit properly, and there would be a bit of a gap when the Vifa's were put in the old SE/R cabinets. I hope this helps. Good luck!