I doubt the Navy is going to just let you extend time in Navy. In fact, if someone here knows you can switch prior to OCS, jump in now. I'll tell you why I don't think it's possible: If you get a pilot slot, you can think about it, and actually turn it down. When you go to the NUPOC interview, and as soon as Admiral Bowman says "congratulations" the navy has you by the balls. You sign paperwork saying that if you're selected at the interview, you're in the Nuclear pipeline.
As for the lat transfers, you need 24 months of commissioned service to apply, which in other words, you need to be a JG. Additionally, since you'll be in a URL, you'll need to get submarine warfare qualified before you can apply as well (there's a waiver for it, I think, but it's not going to make you competitive). The board meets twice a year, once in the summer, once in the fall. I'm willing to bet those guys were done with their initial sub tour...or close to it (you can probably apply during your tour, and redesignate towards the end of your tour).
Also, I've heard of URL to URL transfers where you don't have to wait for the board to convene...Road Program did SWO to NFO that way if he wants to chime in...
I did a lat xfer myself, and I must say, having to wait for what you want to do is very frustrating. I hope you'll be satisfied with subs, because if you're goal is to fly, Power school/Prototype/Sub school/initial sea tour is going to annoy you as you put together your lat xfer application.
Here's the LOI for this November's Board
http://www.npc.navy.mil/NR/rdonlyres/EBC02628-6439-4EC3-8A31-AD5C7C201D3F/0/Nov05RedesBoardLOI.doc