When changing the gearbox oil by draining from the pan, half the oil remains in the torque converter. In the past, I would go through several cycles of draining, refilling, drive a short distance and repeat. Once done then most of the old oil has been diluted with new oil.

If the engine is removed, I take it you have access to the torque converter. Could that be an ideal time to perform a gearbox oil change where you could drop the pan as well as remove the torque converter, and drain the majority of the old oil out. I understand the torque converter would need to be refilled with fresh oil before being put back. And the final gearbox fill amount could not be exactly done until the engine was running.

A non starter or opportunistic? 2010 5.0 SC Autobiography Overfinch in Bentley Silk White pearlescent, 22" Overfinch wheels with 285/35x22 tyres

Megaflush it once put back together. Plenty of places do it now....  Gone to a good home: 2011 4.4 TDV8 Vogue SE Buckingham Blue with Ivory and clear glass = "Rory"
New Defender D300 90
1974 Series 3 Lightweight = "Millie"
My preferred specialist: www.glenrands.co.uk
--------------------------------------------------