I imagine its different where you have an annual bid but where I was if you followed your route you kept it for as long as you wanted. We bid routes for life--or until you decided to give it up.
When a new hub opened locally, routes moved from my building to the new building. A bunch of people followed them, thinking that they would have them till they bid off. The new hub relooped the new area into their nearby loops, higher seniority drivers followed their work and of the 6 drivers that followed, only 2 had enough seniority to keep their routes.
We had 3 routes head north to Rod's building, 2 drivers followed them. I know they were relooped and that one of them had very, very little seniority and I do not know if he was able to follow his work to keep his route.
So for the OP, it just depends.