Should I just change the transmission oil and filter or flush the entire system?

Hello,

I wanted to change the transmission fluid in my 1989 Jaguar XJ40, probably for the first time at around 165,000 km. When I try to shift from neutral to forward or reverse, I hear and feel something. It's not so advanced that the transmission is about to explode; it's still a moderate problem, but it's getting worse over time. If the car isn't parked for at least three months at a time during the winter months, the oil leaks out of the transmission pan gasket.

A 4-speed automatic transmission is installed, which generates an overdrive at around 90 km/h.

The gearbox type is ZF 4 HP 22. Apparently this was installed in many cars in the 1980s.

With this transmission, you have to shift through all the gears a few times before checking the oil level, and then read the oil level in park mode with the engine running. Fill the oil through the dipstick port. This was new territory for my mechanic, as he's only familiar with newer transmissions from the 2000s onwards, from VW, Audi, Mercedes, etc. And with those, you have to pump the transmission oil in through the transmission pan.

He told me he can only get half of the oil out with an oil change because the other half stays in the converter, and only a flush can get the old oil out. And that costs a lot more. And if I just drain the old oil and fill it with new oil, it won't do me any good.

The old oil and the new one are an ATF Dexron 2D

What makes sense now? Just replace it or flush it completely?

Is there anyone in the Lake Constance area who does something like this?