If I change a product's first price and recurring price, this will not affect any existing customers is that correct? For example, if an existing product has a first price of $25 and second/recurring monthly price of $25, and then I either increase or decrease first and second/recurring price for that product, existing customers paying $25 will not be affected, correct?
Yes, you are right. Changes within product billing terms does not affect existing auto-recurring subscriptions.