When I stripped my BW65 box, I also removed the convertor and drained it fairly completely by sucking out the fluid using a mityvac. The pump was also empty as was every pipe, tapping and servo in the box.
None were primed before start up, and it all works ok, therefore I doubt if it will need priming after simply changing the sump gasket, especially as the convertor and pump should still be fairly full.
Mike