I have some differential pairs in Altium Designer that I would like to modify. Is there an easy way to change the track size and spacing once they have been routed?
You will have to reroute the differential pair with the new rules for width and spacing. If you have loop removal turned on, the old tracks will automatically be removed when you place the new pair.
You could easily change the track width using the Inspector Panel, but there's no way to change the spacing automatically. That has to be done manually. The easiest way is to rerout the pair with loop removal on.