Data point: I had a schedule change (~2 hours) of a connecting partner flight months ago. Had to cancel/rebook the return leg (GRU-EWR on UA), which was STW'd via 140 PP's. Because of the prior schedule change, UA.com said a refund was possible and that the PP's would be refunded plus any residual fare. Great, I will just cancel the return leg and use an award to get home.

Took screenshots along the way. UA.com bombed on the cancellation and said to call. Called an agent and they said I'm outta luck with the PP's as it was STW'd, and I shouldn't have cancelled. Told them the website said it was permitted and that I had screenshots. After supervisor review, they gave me back the 140 PP's "as a courtesy", but the residual fare would need to go to refunds. After 14 days, I got the residual as a FFC, so all in all it worked out well - but just be careful if any schedule changes are on partners and the website erroneously says you can cancel. I got very lucky...