I understand that so long as origin/destination are the same or are "co-terminals" of your original destination, and the proposed routing is considered a "legal" routing, you should be accommodated. Sometimes, offering to connect via another city actually helps UA where wx or oversales are concerned and, if that appears to be the case, I've found that a plus when asking for such a standby routing. Of course, if you are trying for the old VDB, you might want to stick with your ticketed itinerary.