Here's my two cents.. I would first off send your customers a survey about where they find value and why they picked yearly over monthly. Next - I would not in

Here's my two cents.. I would first off send your customers a survey about where they find value and why they picked yearly over monthly. Next - I would not increase prices. I would add value, and increase pricing on that. Add more features, and add another pricing tier that people can find valuable with these new features. Continue to do that until you have multiple pricing tiers; (if you don't already). Then add value into those tiers and increase cost to reflect that extra value add as needed.