By seniority- do you mean years of experience?
I know that in the public school sector, a teacher carries years of experience (which affect pay grade) from one school to another, and even from one school system (even from other states) to another, so I don't think there could be any effect in terms of income.
If someone has earned a position of authority (such as, "department head") it's unlikely they'd be the one to be given a transfer, unless they specifically asked.
Does a teacher (non tenured) have a property interest in his/her seniority?
Monetarily, I would say the answer to this is yes- but in terms of prestige or any other benefits that would allow one to either stay in a position or go as they desired, unless s/he has tenure, it's not in their hands- they do what the administrators decide they should do. I've seen it happen many times.
But I'm not a lawyer- and have never had reason personally to explore this issue in any depth.
Maybe you could contact an NEA (National Education Association) rep for a more definitive answer.