Unless it's a single flight number "1-stop with a plane change" then all segments have to clear or you get nothing.

I've found in some cases that if you want to go to the extra effort, you can quote the flight as a multistop. e.g. UA 225 is BDL-IAD-SFO. If you book it as BDL-IAD, then IAD-SFO, it may end up being the same price (well, slightly higher as you have to pay extra airport facilities charges) but it helps improve your chances of at least 1 segment upgrading.