If you want to try this method, be aware that a slip of the razor can very quickly take out more than the stitches. I'd use a good seam ripper instead because I'd have more control over it.
I have 'un-quilted' a several of my quilts and completely done them over. I hate doing it, but there are times...
I get comfortable in my 'spot' on the couch, watch (well, more listen) to a TV show and 'unquilt' for as long as I can stand to. It is tedious, but it takes a lot less time than I thought it would - average is 5 nights for 2 or 3 hours. Bigger quilts mean longer sessions with my most used quilt tool, the seam ripper. Then I re-press the top and the entire sandwich goes back on the long arm.