Dealerships would rather your trans completely fail, since it'll be out of warranty when it happens and then take your money. It's bullshit. All transmissions are serviceable. If there's no drain plug, there's a filler plug at the top or middle where you can suction the fluid out, and pump it back in.
In many cars, the position of the plug often defines the level of fluid needed. Once fluid starts to pour out, that's when you let the fluid settle and stop pouring anymore in. I remember when Audi use to tell me "these ZF transmissions are not serviceable, they have a lifetime fluid". Orly? I knew at 70K miles, my trans on the A6 4.2 was going out, it would violently buck when flooring from a stop and barely had pull when flooring on the highway. The Audi techs kept insisting 'oh yeah, that's normal'. Drove me fucking nuts. So I ditched the car and traded it in for a new 2012 A6, while the old one was still drivable (but I REALLY loved my previous gen 4.2L, it was a beast).
The difficult part was that they were shared cars between my dad and I, and I spent hours convincing him the old one had a dying transmission. Stubborn Russian fathers. I couldn't do shit, since the car note was on both our names, and I needed both signatures to trade the car in. He finally gave in, obviously...and loved the new one, meanwhile I eventually hated it and got my own Jaguar. LOL.
