This has polarised the posters on this thread!

Years ago i would have stuck the "old but with some meat left on them" pads back on - i might have got a few pads wear out of a pair of discs - now the discs/ pads seem to war at a similar rate, and by the time ive worn a set of pads out - the discs need changing too

The old pads are unlikely to have worn evenly - some may have more meat than others and by the time the pads have "bedded in" they are liable to be thinner
by the time you factor in the time taken to fit the discs and part- worn pads, on balance, with a set of front pads only costing about forty quid, i would not bother fitting a set of used pads to new discs

If i had to pay a mechanic/ garage to do it i would supply new pads/ discs, no question

if the pads cost a lot more than that - it might be different but at that price i would fit new pads - otherwise it would be false economy - you can always keep the old pads for next time