Switching won't mess up anything at that mileage. It also won't prevent as much wear and tear - the big question is 'How were those miles attained?' Was the car thrashed when cold? Was it all lots of small trips (engine run mostly before it could warm up and fully lubricate)? If the engine wear is still relatively minor, synthetic will slow it down, just not as much as if you'd started with synthetic at the first oil change.
Switching at high miles is well-nigh pointless, as much of the damage has been done already... Also, the grey-beards tell me that a very worn engine actually RELIES on the grime caused by dinosaur oil breakdown to seal an engine that's worn out of spec clearance around gaskets and whatnot. Since replacing a head-gasket is ALWAYS a b****, most of them won't switch and risk losing the grime that's holding the seals tight (and also robbing the engine of power, of course). But, that's just what the old-time mechanics tell me, it could be a mechanic's myth.