It depends on your level design.. You probably need to use custom obstacles for pathfinding behavior instead of "solids". Or maybe you can disable solid for sprites when there is a bridge over them.
Pathfinding won't work this way, because in the moment of "obstacles map rebuilding" bot won't see the path throught /river/ till touch the /bridge/, and if I disable solid bot will follow throught /river/, not throught /bridge/. Path cost works bad either, because if /bridges/ connects, bot stucks in that point(Coz negative costs(-50) are summarized).
But thanks anyway