re: What should my tenants' new monthly rent be?

Then your rent is too low most likely. I don't know what kind of deal you are running, but a rent to lease deal you generally can't just break and keep their money because you want to up the rent. There is usually a contract and method for how all that works.

Again, what you paid and what you owed mean absolutely nothing. The only thing that matters is the current market and what others are willing to pay.