Yes, alter the payment plan. Old users find out their subscription price has dropped -- that's a happy surprise not an annoyance. Assuming monthly billing. Yearly might be more complicated; perhaps extend the subscription length.
Brewing your own probiotics may be a safer option. People have been eating Sauerkraut for 1000s of years. I just add chopped cabbage and a tablespoon of salt into a quart-sized jar, fill with filtered water and wait a week or so until it tastes sour enough.