Might just be time for a simple decoke and valvegrind. Excess carbon deposits in the head and on the valves can cause preignition, the higher octane reduces the susceptability to ignition in the first place so would delay the onset of the preignition (pinging).

260,000 miles is a stirling effort, and I wouldn't begrudge spending the money fixing an item of wear and tear on what appears to be an otherwise reliable car. (should cost the same as a "plug change", plus a head gasket if you do it yourself. Maybe a day to day and a half leisurely work).

Like any vehicle, if you get a good one, stick with it.