Lament the curse of Kirk. Lament it well. That group of shining young officers, with bright careers in front of them...were totally destroyed by serving under him. Think about it – in 2267, Ensign Chekov reports for duty as Kirk's navigator, a raw cadet aspiring to one day command a starship of his own someday. (That isn't me talking – that's the original concept for the character.) Thirty years later, as the Enterprise-A sets off on its final cruiser, Commander Chekov reports for duty as...Kirk's navigator. In a modern military, he'd have left decades ago for another posting. Uhura did even worse, if that is possible – two ranks in three decades, and still stuck at Communications. (You want to do a sensible re-imagining? Communications actually means 'Intelligence'. She's a spy.)
This doesn't only fly in the Original Series. In the Next Generation, Riker joins the Enterprise-D after turning down a command. Now, this isn't necessarily the career suicide it first appears – as it is made clear that he turned down a lesser posting for the more prestigious assignment. But he turns down numerous others to stay where he is, and twenty years later remains as Picard's understudy. In the follow-on novelizations, he ends up getting a command at last. The rest of the command crew fares little better – all of them still stuck in dead-end postings, when most of them by Nemesis should really have their own ships, or moved to other shore postings. Deep Space Nine was a bit better for this, but Voyager...poor Harry Kim.
I think what I'm trying to get at is that in the navy, command crews change. People are not assigned for set lengths of time and stay there forever. People transfer to other assignments, go off for training courses, get promoted into new positions. Yes, this doesn't happen every five minutes, but it does happen. No-one stays at the same rank in the same job for twenty or thirty years. I quite understand that there is a desire to see the same well-known characters everyone knows and loves in the roles they are accustomed to, but it has a tendency to stifle the character development which is one of the primary reasons for writing a series in the first place!
When I started the Battlecruiser Alamo series, it was with the knowledge that I wanted to make it as realistic a portrayal as I could, which meant getting the science right (thank you once again, Atomic Rockets) and also meant getting the military concepts right. Naturally, things change and evolve over time – the US Navy is a very different place today than it was a hundred years ago, with new roles evolving and old ones fading away. That's the great challenge – and opportunity – of science-fiction, of predicting how the mundane details change.
'Not One Step Back', the fifth book in the series (available at the end of February if all goes well) will be the start of a new 'arc', and yes – there will be some changes to the cast. Some people will be transferred, some people will be promoted, and new faces will appear. That's all part of the fun. Lieutenant-Captain Marshall will remain the commander of Alamo; that much is absolutely certain, and most of the cast will remain in their current roles...but some will change. And some will come on board. And some will leave.
Having said that...don't think that 'leaving Alamo' means 'never heard from again'. Because one of the other things I was determined to do with the series was make it bigger than the single ship, and that's something I am seriously working on for 2014, expanding the scope of the setting. There's a lot going on, with other elements of the overall series 'arc' that I've got projected for the next couple of years – and I'm going to need some of these characters in new places as time goes on. It will give a chance to see them in new and interesting lights – and allow for the creation of new characters, which for me is one of the great joys of writing!