I would ideally do a transmission fluid flush every 30k miles, but you're still fine at 50k. I think you may need to just flush the whole thing, but do be weary of the cheap shops that use these weird machines. Automatic transmissions do release metal shavings which add up over time. When there is too much in the transmission, it damages it and eventually the transmission will rely on the metal shavings. Changing the transmission fluid at this point is no longer recommended as it is so damaged your trans will now just slip. @50k miles you're still clear.
According to a link (can't post url since I didn't make enough posts apparently...), "The typical service interval is 60,000 to 100,000 miles." but " Most manufacturers recommend that manual transmission fluid be changed every 30,000 to 60,000 miles. Under heavy-duty use, some manufacturers suggest changing transmission fluid every 15,000 miles."
According to the official 2016 Scion iA Maintenance and Warranty guide, you are supposed to change out the automatic trans fluid every 30k miles or 24 months, whichever comes first. Now if you want to be supa supa responsible, do the maintenance before the official numbers, so every 25k miles or 20 months (25/30 = 0.8333~, so 24 months * 0.8333~ = 20 months)
Just dropping the pan will not change out all the trans fluid. You'll get a small percentage of it, I'm not sure exactly how small but it's not enough. Get a professional to flush it out properly and make ABSOLUTELY sure they clean their flusher every time. Pay a premium if you have to. Don't cheap out on this. Going out of your way to baby your transmission will make you more than likely to squeeze a ridiculous amount of miles off it.
Lots of automatic transmissions die out early because people don't think to change out their trans fluid. Our car doesn't come with a trans fluid dip stick which sucks because with it you can quickly check the state of your oil. You can spread some on your hand, if it's liquidity and clear with no burning smell, it's clear. If it smells like burned crap then you need to change it ASAP.
I don't see any mention of what transmission fluid to use, so we'll have to research more on that. Your dealership should know.
My car is personally at 14k miles at this point, I think the dealership changed out the transmission fluid since this car has been owned for more than 24 months by some other dude I bought it off from.