Can't tell you if it can be done myself as I've no knowedge of relative head bolt spacing, cylinder barrel thread depths and fasteners, length of pushrods, relative valve geometry and lift of camshafts etc between 500s and 650s, although there'll be many who know in this amazing corner of cyberspace.
But what would happen to the compression ratio, assuming it could be done? I have no idea but I'm assuming it would reduce, and the more so if you were to chamfer the lips of the bores to eliminate the step? Or were you thinking of skimming the head too?
What I DO know, ref another period marque, is that while some 650 heads will go on some 500 motors the straight consequence is a comp ratio of 6 to 1. Don't know about pre-ignition in that context - probably doesn't even get hot enough in there! Not saying the same applies with BSAs, not at all at all, but my hunch is, even if the bits can be made to fit, that it's a not-great idea. Very happy to be told that's rubbish though.
Just speaking personally and generally on (or even slightly off) the topic, I think 'tuning' is a fraught area. A machine has to be treated as an entity. Starting with a 100% verified top-notch bottom end, then looking at the piston and cam options, and the valve gear as a whole, and then thinking hard about carburation and ignition timing, gearing, clutch options etc etc etc and trying to end up with something that'll hang together. And what about the brakes . . . and the rest of the cycle parts . . . and the lights . . . etc?
Having recently acquired a weird beast of another marque where someone had tried to extract more power than was probably wise, had failed miserably and then sold the thing in disgust to a sucker (me), I have had to get interested in these things. In this case, not being in a position to steal the food budget for 'yet another mechanical misadventure' as it was put to me, I used what had been done as the basis. But I had to start again at the crank and spend a lot of time, and some money (from my grape-juice budget, natch), to make it work. Which it does, for the moment. But it's lucky I have a trailer - and a dustpan and brush to sweep up the bits when it lets go.
Most things are more rewarding long-term in - or close to - the state of tune they were designed to be in, and I would have thought, not having one myself, that an A7 is right at the top of the list of most-rewarding machines ever made (although my B31 might argue). The ones I know are just tremendous. The sanest classic route to more oomph is cubes, which says 650 minimum and probably says 750 (so later Bonnie or Commando really, unless you have the appetite to face an Atlas or an Interceptor). While it's nice blasting around with 40-odd bhp or 50 even, it's equally nice with a lot fewer, it's just different.