Weather delays and nice agents aside, its best to make sure you take care of things properly on AA's side before going to the UA counter.

First, ask them for an endorsement. They may give you a straight endorsement, or endorse with "face value only." Either way will work, although the latter may mean a difference in fare.

Second, ask them to keep your reservation open in case you still need to take the flights. They should be happy to do that for you if you are pleasant.

I've found AA is more cooperative in this situation than NW, and UA has been able to take the tickets far in advance, with me paying the slight difference in fares.