+1 - Successful outcomes to these situations require a lot of legwork on your part. It is far easier for UA to tell you that there's nothing to be done and offer you a refund than to spend the serious time to figure out a routing which works. This is especially true because to many agents, these are simply airport codes and they have no idea where they are, so logical thought about reroutes is hard.
Your best bet will be routes served by UA. That is because UA is more likely to open up its own inventory (although not a given) than another carrier is (why should they help you out since you're not their customer).
If you sketch out options, including carrier, flight numbers and the like, and those flights are available, UA is more than likely to do them for you. Backups would be the use of other *A carriers.
Don't forget that if you are flying USA-IST-XXX, depending on the location of XXX and your departure point xUSA, it's sometimes possible to route TPAC (in a reroute situation).
Good luck.