I just learnt that the 2013 onward 200s are based on the 250 engine, whereas the earlier 200s were based on the 125. What is the practical significance of this changeover? If I was hovering between buying a 2012 and a 2013, how important would that change be?

The early 200s were sleeved down 250s.  Later models had plated bores but I don't recall any mention of being based on the 125.  Presumably the difference might have been in the gearbox ratios if so?

Like the previous poster I am pretty certain that the 200s have always been sleeved down 250s. I had a 125 apart (07 or 09) not long ago and I doubt if there would be enough metal to take it out to 200. The sealing surfaces are minimal as they are.

The pre 2016 models were based on 125 but the new ones are based on the 250, using 250 crankcases and a flywheel made for the 200. Remember reading that on the beta site.

To the best of my knowledge all the Beta Rev 200 models were based on the Rev 3 250 engine fitted with a 125 con rod and sleeved down cylinder, the Evo may have been different though.

I have a 2008 Rev 3 200.  It is a sleeved down 250.  It now has a 250 top end.  The piston and cylinder are different plus there was a bush in the small end of the rod to reduce the diameter to the 200 piston pin size.  The 200 uses the bushing as a bearing so has no roller bearing in the small end.