Is it legal to change around the sequence of continents after the trip has begun?

For instance, if I have a SYD-LAX, LAX-UIO, UIO-LHR, LHR-JNB, JNB-HKG, HKG-SYD; I fly to UIO, then I decide I don't actually want to go to LHR right now, I want to go to JNB instead. Can I flip the routing to UIO-JNB (via GRU, I guess), JNB-LHR, LHR-HKG? (after flying the first segment, so hypothetically pricing should not change?)