If you want to make more - and the reasons for that can vary from pure greed to absolute need - experimenting with pricing can have the best effect, as every extra dollar MRR compounds over time, and it is often easier to get 10% out of customers than 10% more customers. The patio11 "charge more" advice is an easy place to start, and if you grandfather in all old accounts, there is minimal risk in doing so.
But it is your business, so do what you think is best.
I offer an annual discount for a product of mine, primarily because for a one year plan it only costs me a single transaction cost from my payment processor instead of x12 transaction costs.
Most obvious recent example is to search online for JC Penney’s pricing strategy fail, where a prominent Apple executive had the same exact thought you did, and completely alienated the JC Penney shoppers who actually like seeing a price tag say $100 and then get surprised with various discounts at the cash register and see it be $20.