Replacing disks keeping pads?

Can't see why not assuming that the discs aren't deeply scored. If they are then the pads will be too, and will take ages to bed in - and will re-start the scoring in the new discs. Should be safe, but less than ideal - and your braking effect will be reduced during the extended bedding-in period.

Pads are, what, £15, maybe £25? While it's all in bits you may as well replace them, save a job in a year's time, and know it's all good.

I would personally change them. Not only for it being good practice, but to keep your braking performance tip top (not that it will be awful or that kittens will be killed etc.)

Years ago my dad did this on a MkIII Orion he borrowed at which he threw far too much money at to deal with the usual suspects. The braking performance was OK (but not brilliant), but despite using copper grease to silence the pads in the relevant areas they did not half squeal for what I remember at the time seeming ages!

It's perfectly fine to just replace the disks if the pads are OK. I've done it before absolutely loads of times and had absolutely no issues whatsoever.

If the pads are more than half gone then you might as well change them while you've got the calipers off, but otherwise just leave them be.

I shall admit that the pads on the Orion came off a worn set of disks which were probably very grooved (You could hear them hum under heavy braking!). I am sure he tried to wet/dry them but in all honesty the discs should have been changed first time around...

I once used second hand pads, as the ones on a car in the breakers had loads of meat on them, i wasn't made of money, and mine were almost dead. Got them for nothing, yet they were genuine VW ones, so i thought why not.

Before fitting them, i gave them each a few inches along the top of a kerb, to clean/cut the face of them. Needed to ensure it was perfectly even, so just did minimum.

They were absolutely fine, worked great, did me well for many thousands of miles. Probably better than buying new cheapo ones.