For a hassle free experience, new pads.
If the rotor has worn to a uniform thickness you’d probably be OK with the old pads, but I’d scuff them up with sandpaper.
If the rotor has worn so it’s thinner in the middle (which normally happens) then 100% new pads time. The pads wear to match the rotor shape, hence, the pads matched to a worn rotor will ‘bulge’ in the middle. Installing a new rotor will mean that only the middle of the pad will contact the rotor. At best you’ll have poor braking for a while. At worst the pads will glaze over and howl, and have poor braking. Well, at worst you’ll get hit by a bus….