I'd imagine many earlier BTR's would be fairly worn out by now, either clutches, or even just the second/fourth band?   Not to mention potential gummed up solenoids, or valvebody?

interesting, 
A mate of mine's Dad's AU ute has an EL trans, and he's about to fit an AU trans in an XG (using XH transmission loom) 
wonder what's actually different. i'll reply here if he finds any issues with this combo. (assume the AU with EL trans used all the AU stuff though) 

Speedo sensor, gearshift indicator (sometimes referred to as neutral safety switch) and wiring loom, only external, internal wiring doesn't need to be changed

what electronics do you mean for this Thom?

   

All au trans will be a straight bolt in, just make sure you use the electronics from the car as one of the years has a different gear position indicator and I can't remember which one

Ok so b series transmissions are a no go? Ill see if I can get one from an au, any difference between year models?

Ea to au btr's are mostly interchangeable, in 6 cylinder cars with another 6 cylinder btr (some v8's had sports shift like ed sprint, el gt, au ts50 etc) to use an earlier btr in an au it is recommended to fit the au bellhousing to the e series transmission (straight swap make sure you get the converter re seated correctly), the e series bellhousing wilbolt to the au engine but it won't bolt to the sump, you'll need to use the au speed sensor, neutral safety switch and wiring, ba an later btr's can't be used in a ea-au (except maybe column shift ute transmissions but im not certain on that one) as they don't physically have shift positions for 1st to 3rd and are setup to run with the "sports shift" shifter, also unless it is a ute transmission it won't have a mount for the speed sensor (without it the transmission won't shift) as ba to fgx (except ba utes) all came standard with abs and use the abs sensors to tell the ecu how fast the car is going for controlling shift points

Yes cleaned and increased stall a little.     I thought that too, bought same one a decade ago for another car I owned and had not fitment issues. I wonder if the convertor was not seated properly on the flexplate and caused the whining noise?

I'm not sure if this is ok on this forum because its for an AU so let me know if I'm in the wrong place. Recently a mate killed the BTR 4 speed in his series 3 AU and he wants to pay me $500 to do the job. Now I am wondering what was different between the btr's between the models. I found a box out of a BF that is supposedly in good condition but I'm not sure what if anything needs to be changed to fit it to a AU. So what boxes are compatible and what do I need to keep from the old box? Also is it worth it to rebuild these gearboxes or am I better off just getting a new one?

That last pic , assuming converter was reco or cleaned on rebuild ? shows signs of something going on with the front bush , if it has picked up or damaged the bush could contribute to a noise  Rest looks fairly normal. Bit odd the studs are catching in the flexplate