Good Morning Almostdone.
Welcome to The Club!
You can never change out the transmission fluit too much, IF you drain and refill without a pressure flush!
If you remove the drain plug and allow the car to sit for two hours you can get as much as six quarts out of it.
Four will come out in a rush, so if you are in a hurry just replace what you drain (bringing it back to the fill line.)
A Subaru factory in-pan filter is only thirty something dollars so its alot cheaper than a tranny.
As far as the aftermarket filter goes, just make sure that you have the flow the correct way, or you can burn out the transmission.
Are you coming to Reading?