For Fairfax: I suggest calling the world languages office, as this is really a question for them. Their number is on this page:
https://www.fcps.edu/registration/dual-language-immersion-programs-registration#k
The website used to list the number of open seats, if any existed, at each immersion school. However, I don’t see that information now. If I were you, that’s what I would ask about, and then you’d know which school might be the best shot. People do drop out over the summer as some out of boundary families realize they won’t be able to make the commute work, or for other reasons.
I would be surprised if they give any special treatment for someone coming from an immersion program elsewhere.
I have children currently attending an FCPS school with an immersion program, and it’s a great program. Best of luck.