The OP changed the filter 15K miles ago so it should be ok to leave it in.

My $0.02 is exchanging the fluid using the cooler lines is fine IF you don't damage the threads on the radiator whilst removing the cooler hose. It is essentially no different than how the fluid moves during everyday operation of the car.

Flushing in my opinion/understanding is sometimes done with a chemical cleaner, under pressure from a machine and with the possibility of pushing the fluid in the direction opposite normal flow. This flushing scenarion could cause problems and is different than a simple fluid exchange.

Since this Benz has a pan drain plug and a converter drain plug I'd probably just drain them and not mess with the transmission cooler lines.

Good luck!!!

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