Applying internally for positions?

My boss has invited my to apply internally for a position reporting to him, that would entail a promotion and a change of responsibility. It would be a stretch role. I'm curious to hear if there are any pitfalls, or things to be aware of when it comes to applying for an internal role.Last thing I would want is to apply for the role, not get it, then be marked as "unhappy in my current role." All thoughts welcome. Thanks!

Edit: I should also note that he said “I’m worried about you leaving and want to make sure you’re fulfilled.”