In Puerto Rico, does it make sense to shift to a corn-based strategy mid-game?
It makes sense to take Corn at every point during the game.

As you point out, Corn requires fewer colonists to manufacture and is worth the same amount of VP tokens when shipped. 

It actually doesn't make sense to go for the higher yielding cash crops in the beginning of the game, until you have one or two other crops to protect it. Imagine only having your cash crop, Coffee or Tabacco. If you choose Craftsman, many if not most of the other players are likely to choose Captain and force you to ship your cash crop, since you will benefit the most from a Trader phase. Taking an early Corn helps protect your cash crop from being shipped during the Captain phase, and usually it will require two other crops.

Mid-game, you should still be focused on setting up your economic engine. A Corn (or any other good) is helpful in emptying the Trading House so that you can still trade in your cash crop.

Late-Game, every Corn you can ship either before your opponents on a boat that will not empty (if you have Warehouse for storage), or on a boat that will fill before your opponents can place Corn is VPs gained on your opponents.