Can you change from surface fleet to submarines?

If you're insane enough, absolutely. I had many friends (well, not MANY) who were originally Surface Navy and volunteered for Submarine Duty.

The Submarine Force is all volunteer; in addition to a relative rating (i.e., not surface specific, like a Storekeeper or Boatswain's Mate) all you need to do is volunteer, and pass the subsequent physical and psychological testing. Aside from the minimum Administrative crew complement (Admin, Medical, Supply), submarine crew ratings are typically specialized in either Engineering, Electrical, or Electronic rates these days. Even the Torpedoman and Quartermaster rates have been combined into Machinist's Mate and Electronic Techs.

Submarine duty is considered hazardous duty, and members are compensated with hazard pay. Most guys I've known who've made the switch have enjoyed it, since the crews are smaller and more tight-knit, and the professionalism is a lot higher as well. Also, in the Submarine Navy, your respect is earned by your ability and qualifications, not necessarily your rank. A senior enlisted who is unqualified in submarines is just a FLOB (Free Loading Oxygen Breather) and is no different than any other rank that isn't qualified. A junior enlisted member who is Qualified in Submarines is considered more valuable. Rank is still respected of course, but within the crew, overall respect must be earned by ability, actions, and hard work; you don't trust anyone who can't cut it during an emergency, since you don't have as much time to respond and contain a problem like surface vessels do. I remember one Master Chief Radioman who made the switch from Surface to Submarines and felt his rank should give him everything without any work. He lasted one deployment - the CO fired him after he lost the respect of the entire crew.

It isn't easy by any means - but I wouldn't trade the experience for anything.