On 2/7 I booked flights DEN-PVR-SFO in May/Jun in F (well technically C since it's Mexico) both 738's.. At the time and even right now it will only display the first 2 rows of F to select seats from even though it shows there are 4 rows..rows 3 and 4 are there but shown as unavailable on United.com.

However when I look up the flights on ExpertFlyer it shows both flights as 738's, but they only have 2 rows of F..3 and 4 don't exist. The economy seat map is normal.. I was thinking maybe they are considering a swap to an A319..but from this thread it's happening to a lot of flights.

Also, UA is appears to be only willing to sell 8 F seats on these flights even though they are 738's with 16. Quite strange.

The flights are:

5/28 DEN-PVR UA 1944, 10am (but same thing is happening on UA537 at 8am that day..also a 738)

6/1 PVR-SFO UA 1243, 4pm

I suppose the 2 rows of F is the safety net for an unknown schedule based upon the fleet with the A319 being the smallest plane they would put on those flights (if I get screwed by an RJ swap I'll be changing my route).