you guys are hardcore. fortunately for me, i got used to G4 rackets and i can only get G4s here in the US. plus an overgrip and i'm good.
Future calling. Since I couldn't find so many threads about this 11 years later, thought I would just give my two cents. I've used a file and a vice, just carefully scrapped each side just a little bit at a time. Measured between, no problems. Had an airpressure to blow away the scraps between each side, and also to keep to file fresh. First one took 20 minutes, and went from G4 to G5, perfect fitment. Did two rackets, and second one took 10 minutes, G4 to G5. Theres just no way you could fail this if you do it carefully. Much easier then exchanging the whole grip with remove, fit, glue and so forth. Sand paper will probably ruin the sides, so I would definitly use a file, both faster and much more accurate.
Used 300 grit sand paper on a G4 --> G5.5? It takes some time, but as long as you just let the paper do the work, you can get a pretty good feeling grip. When i did this on 2 of my racquets, both G4, one was slightly noticeably thinner than the other one. I sanded the bigger one down a bit more to get closer to the other one and it feels great. Note that this does smoothen out the bevels on the racquet and takes a bit of time. They should make more G6 racquets because I would definitely be buying them.