Would changing itinerary affect last segment?
 

We are in Chiang Mai now. We may be able to change two flights for a later flight to our destination with Thai airways (not with united airlines whom we booked the awards ticket with) On our way back from our destination we would be on Thai airways for the first two segments but with air canada on the last segment.If we do change our flight time would our last segment of the ticket with air Canada be intact or will united airlines cancel it?

Is everything on one reservation? I would think if you're able to change a UA-award ticket with Thai, United will still see those changes on your reservation and will record you taking those flights. So your AC flight won't be cancelled. Maybe someone with actual experience doing this can chime in.

Everything needs to go through UAL. Don't take anyone's word on here that someone will magically monitor your itn.

Again --- You need to call UAL res, or you may be eating Pad Thai for the next few months!!!