this is what i recently did on mine.
bit of routine maintenance.
(they were replaced september, so not exactly old)
drilled out all vent holes, with drill and 4.0mm bit.
if you hit something hard STOP drilling and move else where. Sometimes Porsche will fit a weight inside the venting area in the middle to balance the rotor. nothing to worry about, just carry on drilling another hole.Some discs have them some dont.
Its like a very thick spring/coil, not disimilar than a big version of what would hold the two halves of a clothes peg together. It would appear they compress the thing, insert it , and they lock into place under tension.
My first set of rotors , (now removed) had one in. This set doesnt have any.
take calipers off (should be able to leave the pads in the calipers i did0 you can inspect them in place, just speeds things up abit.
tie caliper up out way, not to stress brake pipe
remove discs, flush out all the loose crap from drilling.
clean hub and hat face, with wire brush,
better still i did mine with a wire brush on a drill.
nip up the hat to rotor bolts (make sure they are stone cold)
I did mine with a 1/4 drive socket set and little T bar, as tight as they go. I got about 1/4 of a turn on each bolt.
(they had been on the car about 5K miles, heating up, cooling down etc)
also roughed up (deglazed if you will) the rotor surfaces with a real corse emery cloth, just a good rub all over.
refit.
only really did this to mine as car was off the road for rear caliper painting, seemed ideal time to clean them up, as i waited for paint to dry..