Let’s say Stripe would let me 
Change the price of existing subscriptions | Stripe Documentation Or at least, I’m not entirely sure but it looks like it would. I can see how this would be open to abuse, but I suppose that is what laws are for!
Changing prices
Subscriptions can be changed by switching the price that a customer is charged or by changing the quantity. Normally the result qualifies as either upgrading or downgrading the subscription, depending upon the price difference between the two subscriptions.
Assuming a customer is currently subscribed to a basic-monthly subscription at 10 USD per month, the following code switches the customer to a pro-monthly subscription at 30 USD per month.