Even within NYCT, there are different unions which previously claimed that work could never be moved from one bargaining unit to another. A good case in point is the Q32...
Until 1994, the Q32 (previously M32, previously FACO/MaBSTOA #15) was all OA work based in Hudson Pier Depot.
In 1994, the Q32 was split between Hudson (later Quill) and Casey Stengel, meaning that some OA (TWU-100) work was given to TA (ATU-1056).
In 2010, the Q32 was consolidated in Casey Stengel, meaning that more OA (TWU-100) work was given to TA (ATU-1056).
Along the way, other routes (M22, M35, M50, M60, M66, M72, M116) have flipped back and forth between TA and OA.
In each case, the arbitrator ruled that the work does not "belong" to the union at all — it belongs to Transit, in keeping with the "management rights" clause of each contract (in essence, "management reserves the right to manage"). Given this precedent, it should be relatively easy to swap routes within MTAB (e.g. Q53 in Far Rockaway Depot, Q60 in Baisley Park) and, eventually, between MTAB and NYCT (e.g. B84 in Spring Creek, BM1/3/4 in Flatbush, Q18 in Grand Ave, Q29 in Fresh Pond, Q32 in LaGuardia, Q110 in Queens Village).