Usually, an airway that takes you whiggling along the axis of the semicircular rule has a defined odd direction which remains valid even if a segment of said airway is a bit off. This prevents ATC having to issue new levels to everyone. Only if the airway ends or you join another airway an even/odd level change may be required. Today most of the time airliners fly direct routings that take them off the traditional airway system so ATC will tell you when they want an level change.