I wouldn't do it. The button rewind without self timer is prized nowadays for the major right hand gripping real estate. Maybe I had one film rewind snag for not keeping the button depressed. Every time the camera is opened for some purpose there can be problems. My last CLA was on an M6 that had to go back twice for further minor adjustments. Certainly you are right to wait at least until a CLA is needed. Swapping out a body shell for a lever rewind I would definitely not do. My button rewind M2 was serviced by a German in 2007 and it has been better than ever, but only minor things were done and the reassembly was perfect. I am sure he would refuse to do what you are thinking of. The nice black paint might inadvertently be scratched, which I wouldn't like.