Between myself and my brother we've sussed out how to edit the save-games, following a nasty incident in which a "new command" led to a Porpoise. The files below are the key ones, editable in notepad. I've also listed the elements of each that are the bits you'll be interested in.
Activeuserplayerunits.upc controls the hitpoints and efficiency of each part of your submarine. Also each individual crew-members experience, skills, specialisation, local skills, hit points etc. It also includes which Submarine you're currently captaining.
CareerTrack.upc controls your current amount of renown (useful for those of us who aren't skilled enough to play at 100% but would like to afford decent torpedoes
), tonnage sunk, medals, available medals for crew, available promotions for crew, and all the information held in your Captains Log.
Playersettings.cfg.rep controls your current date, some further submarine captaining elements , and what radar, guns, AA etc you have installed.
I've not personally edited which sub I'm in, so am unsure if it's Playersettings.cfg.rep or Activeuserplayerunits that determines your sub class.
Hope this is of some help.
Edit to add : I'd very much recommend saving in port and editing that one, rather than a "live mission" save. And naturally backup.
VB