I would suggest you against this change. I will not compare OpenVZ and KVM, as this would be useful only if you have not yet created a server.
The cheapest Digital Ocean droplets come at the 5$ and 10$. Both of them can handle the listed tasks. It is really good to think: how much are you going to save if you migrate to OpenVZ? Probably not more than a few bucks, but this may also cause problems:
Can you handle a successful transfer? This is not a "drag-and-drop" process, so what are you going to do if something brakes? Is the time, that you are going to spend on transferring and fixing possible issues justified?
What are you going to do if the OpenVZ server delivers less performance? (I am no talking about KVM vs. OpenVZ in general.) My personal guess is that a company offers OpenVZ plans, which are cheaper than Digital Ocean KVMs, they are running too much containers on a single server (and that`s not good at all).