A particular subclass of this problem: the person who heads up your (pick a department randomly) Marketing from e.g. your seed round through your Series B is a) likely to be young and on-paper unprepared to run a Marketing department with 10 direct reports, b) likely to have a perceived lack of gravitas, c) likely to lack industry connections. Your VC fund and/or senior management may decide to bring in someone More Appropriate For The Role when you need a Director of Marketing.
This presents a dilemma: what happens to the person who built your entire marketing strategy when someone has just been brought in to own the entire marketing department going forward? Is new person going to report to them? No, that's an obvious non-starter. Are they going to report to new person? A surprising number of people would prefer to not have perceived issues with the existing team (most of whom were, after all, hired by the existing employee) deferring to that person rather than their new titular boss.
This is a time at which you either pray you are listed as a founder of the company or the founders are willing to go to the mattress for you, because otherwise the outcome is "Thanks for your service; best of luck in your next job."
Even if you do end up being replaced, you can manage that transition so that you can parlay that experience into a more senior role at another company. If you have a good relationship with the founders (a necessity of any startup), they should give you plenty of heads up and help in navigating this, even if they do decide to bring in someone with more experience.
This is also related to how engineering is often viewed (correctly or not) as a young man's game. The most commonly cited reason for that is how there's always new stuff to learn, but even if there wasn't so much churn in the relevant knowledge, the fact that younger employees can grow and "catch up" professionally much faster than in other business functions plays a role.
The problem is that you're a known quantity, and the founders/investors see you as a great worker -- in a certain role. They perceive some problem, and since you're part of the system with the problem, you are seen as incapable of fixing the problem. Better to hire Magic Mike from Facebook or Brilliant Bob from Google to come in and make everything better.
I've seen this happen so many times. If you're an engineer, your choices are generally a) swallow your pride, ride out your vest and don't advance in your career, or b) quit, lose the money, and hopefully move into that more senior role somewhere else, so that you can be Magic Mike next time.
The idea that just hiring a team lead from Amagoofacesoft will solve all your engineering management problems is deeply embedded in the Valley ("nobody ever got fired for going with IBM"). Never mind that you'll be scraping the bottom of the middle management barrel at those companies, since the really good ones aren't going to leave.
Startup engineering is a young man's game in part because it's harder to fool more experienced engineers into drinking the koolade.
But the way options work they are incentivized to stick around when it doesn't make sense and penalized if they do want to move on. That same early person probably does not have the capital to exercise their options when they leave.
Seed round: "Marketing Bob, you'll be employee #2. 1% -- 4 year vesting, 1 year cliff. You'll do everything from writing blog posts to attending conferences to talking to every single customer."
B round (2.5 years later): "Marketing Bob! Great news: we're now worth a billion dollars! That owes an incredible amount to your heroic efforts! You have excellent ability to become a co-founder at your next gig! Which starts whenever you want it to! We strongly recommend you start planning for it right now, and we'll even clear your calendar for the next two weeks to allow you to devote all of your efforts to planning! We hope you are socially aware enough to understand what we are saying here!"
Marketing Bob may well be CEO Bob at his next gig, but he'll be entering it being a) fired and b) having had ~$3.75 million stolen from him. (One would hope that the co-founders would go to the mattress with their VCs when the VCs say "Come on, vesting, we all know how it works. Of course the employee stock pool gets the 37.5 basis points back. That's like twenty engineers -- we need every point now!")
It's 25%/year with 4 year vesting (so 25 + 12.5 for the 1.5 years left)
Much more often, there has been agreement that there is a need for a new layer of leadership that we did not have before, and then we have to decide whether we promote or hire on top of someone. When we have had to hire on top of someone, they have taken that option gracefully while gracefully also keeping the package of other options that they were handed when they joined.
"Report to the new person" works surprisingly well if you sell it the right way.
What I'm thinking about is basically transferring the options to the new guy (so he also gets more stuff in the future once it's vested).