Beyond just an upgrade to something lighter or larger, rotors wear out over time. Many discs will have a minimum thickness printed somewhere on the rotor; you’ll need a set of calipers to measure how thick yours is. If your rotors don’t specify a minimum, measure the thickness of the braking surface and compare it to an area the pads don’t touch; if there is a difference of a 0.2mm – 0.3mm your rotor is worn and it’s time for a new one.
If you don’t have a set of calipers to measure with, take a pick or a paperclip and run it along the rotor; if the pick gets caught up on an edge at the bottom of the braking surface, it might be time for a new disc.