This process which needs done will almost certainly create pain on both sides but its a procedure that needs undertaken otherwise they will always dominate you as a work force. If they take away the 5/4 roster, given your already high on hours.... What could go wrong eh
the history of the 5/4 is because they needed it to stop crew getting high on hours.
Internal flights and shorter sectors on the continent mean't a lot of bases would be more optimum for them on a 5/3. Force them to the table under your terms, not theirs. Good luck.