Sorry for reviving this thread. The discussions above seem to center around partially-flown return trip. I'm curious what about partially flown one-way trip?

For example, if I book a one-way trip from A-C with a layover in B (i.e., A-B-C), will I be able to change the B-C leg (either date or destination) in any way after landing in B?

This sounds similar to skiplagging, so I wouldn't be surprised if the answer is no. However, I'd still want to consult the experts here just to be sure.