The first flight of my first MR, just to set myself up for the 6 RTs of the nested MRs for the FLY3 in April, got cancelled due to a mechanical.
The best I could do timewise was a re-route through ORD (instead of straight BWI-DFW), so in that case I didn't want original routing credit as I was taking an even more scenic route. I did have to spend about an hour combined on the phone with AA reservations, at BWI and ORD, to cover all the routing changes, moving my first RT back two days, moving my return from the MR back to accomodate that extension, and switching the origin of my second MR RT to reflect having to start from DFW instead of DAL (as I should have been set up in already in the original schedule). I had all the flight info on a list, and decided how to ask for the accomodation which would both affect me the least and be the simplist to get across to the CS reps on the other end of the phone. Somehow I wasn't on the passenger list for my last FLY3 RT back to DFW, but the GA didn't have a problem putting me back on.
I did my RTs as DAL-AUS, both for the short flights but especially because it was the same plane turned around, so I could not miss connections. That and having a very soft back end to accomodate such moving back of flights (fronting a week vacation at a resort) meant that it was a low stress weekend after that initial hectic Friday morning.