R-401A runs a little lower head/hot gas temp so I like it a little better than R-409A. Both work fine though. No need to change TXV. I use 100% alkylbenzene oil. MP39 is claimed to work just fine with mineral oil (IIRC).
have used R409A and R401A without changing TXV. both work fine
Changing the cap tube is still the best option since you're assured of "fresh from the factory" passage of refrigerant. If they brazed the cap tube to the suction line as a heat exchanger, duplicate it by making a new line set. The cap tube doesn't need to be brazed to the suction line. Soldering the lines together will serve the same purpose, requires less heat and far easier to apply.