Let me explain a bit more. I don't suggest raising prices for early customers who are receiving the same product they signed up for originally. They can stay at the same price, which is grandfathering them in.
More typical though is Customer 1 signs up in 2015 when the product had features AB. Then in 2018 the product adds feature C. Is it fair to raise prices on them? Maybe, maybe not. In this case, I would still recommend today doubling prices for all new customers. You can go back to Customer 1 and say either:
1) we raised prices, but since you are an early supporter you get the current price for life with features ABC (if feature D is added, don't give it to them for free, they have to upgrade to the new pricing).
2) we raised prices since adding feature C is providing more value, but as an early supporter we are going to keep you at the original rate for 1 more year.
More typical though is Customer 1 signs up in 2015 when the product had features AB. Then in 2018 the product adds feature C. Is it fair to raise prices on them? Maybe, maybe not. In this case, I would still recommend today doubling prices for all new customers. You can go back to Customer 1 and say either:
1) we raised prices, but since you are an early supporter you get the current price for life with features ABC (if feature D is added, don't give it to them for free, they have to upgrade to the new pricing).
2) we raised prices since adding feature C is providing more value, but as an early supporter we are going to keep you at the original rate for 1 more year.