Umm, OK although I'm not 100% sure.. again I need to hear back from Bert (at least his advice), but, I've converted the Graflex back on my 4x5 Series D to a Graflok back using an adapter kit that Bert sent me. It's 100% reversible, and it takes less than 1 hour to do. You're taking off the Graflex mount that's attached to a rotating plate and attaching a Graflok mount. Pretty straightforward stuff.
My guess is that it's a pretty similar situation for a 3x4 to 4x5 mod, in that the thicknesses of the 3x4 Graflex mount is probably not that different from that of a 4x5 Graflex mount, and if the rotating plate on the 3x4 is at the same or very similar distance to the film plane as is the rotatin plate on the 4x5, then it's just a matter of fitting the larger 4x5 mount on the smaller plate.
Now, if there is a slight amount of 'shimming' that's required, well, it's an SLR, why not shim the ground glass? There's room enough in the GG mounting area for adjustments, at least the mm range.
The only thing that a conversion is going to do is to *add* to the distance between the flange and the film plane (i.e., the modified 4x5 film plane ain't gonna get any closer to the lens; if anything, it's gonna get farther away.)
What that means is that to achieve proper calibration in the focus, you just need to set the ground glass an appropriate distance away from the mirror. Again, such a modification would be easy (relatively speaking) and 100% reversible.
The only non-reversible modification to my eyes with John Minnicks's conversion is the two semi-circular side notches he makes on the sides of the front to accommodate the two knobs on the front standard to allow for tilt. I think that's a fair tradeoff.
What would you prefer? A 3x4 that doesn't get used and stays on a shelf in all it's original glory, or one that gets heavily used albeit some relatively harmless modification?