I highly doubt that very many would, especially if they are paying out of pocket(ie not for business). It's much more effective(with regards to getting upgrades) to book flights during off peak times and to pick flights with the most available F seats. You check the number of F seats by going to flightstats.com and searching flight availability.

I just searched LGA to MIA and the 1:45 and 3:22 have 7 seats in F, while the 6:50 has 3. Much better to pick the earlier flights or SDC to them in this example.

There is one other thing I forgot to mention. A higher fare class makes same day confirmed changes much easier, as the higher the fare, the more likely the availability.