I've got to disagree that "the chances of it happening with a full F cabin are probably less". These Airbus switches normally occur for mechanical problems, and UA isn't going to note that row 3 on an A320 has some pax seated in it already, when the alternative is to cancel the entire flight for lack of a replacement aircraft other than a 319. The 320 to 319 switch is particularly seductive to crew scheduling, as UA pilots are cross-qualified in these two aircraft; in many other situations you also have to search for a new captain and first officer.

My track record has been about twice per year having been scheduled on a 320 and ending up on a 319. I never, ever accept seats on row 3 of a 320 for that reason, unless there is absolutely no alternative, and then I remain nervous until we're off the blocks!