However going EWR-IAH-MSY when one is tkted EWR-MSY wont be able to be done as its a change of routing and they usually (anything can always happen, just dont count on it) wont allow you to SBY except for the way you are tkted.

One can do SBY say on a NW or DL flight that also has a CO code-share, as long as its still the same routing. So a EWR-DTW can be done on NW even if booked on CO as long as that NW flight has a CO code-share flight # to it. Going EWR-MEM-DTW is not allowed even if it has a CO code-share # as its a change of routing. The NW EWr-DTW doesnt have a CO # to it then no SBY allowed