All of the fluid gets cycled/mixed together.

I normally recommend installing a drain plug and do a follow on drain/fill about 5k miles (or next oil change) after the tranny service. Done at the proper intervals that's more than sufficient to replenish the additives.

Instead of installing drain plugs, I use a vacuum controlled fluid evacutor to suck the fluid out through the dipstick, then refill. Nice and easy.

Lol or you can use my most recent method of having a trans cooler line come off while driving and shoot 4+ quarts of fluid everywhere! Hahaha but I'd recommend not doing that...