My understanding when I presented to by our rep is their is a higher cost for Robot/Studio when using Cloud Orchestrator because you are not paying for Orchestrator, where if you used Enterprise Server there would be a yearly cost associated with it. (Keep in mind, the information that I am aware of was before they made changes to their licensing model)
Someone from UiPath would have to speak to the upgrading of license types, I would suggest reaching out to your Sales rep directly for such information as I imagine the pricing model would be slightly different from customer to customer for base prices and potential volume discounts and such.
As for connecting to Orchestrator Cloud with your existing Studio. Yes I don’t see why you wouldn’t be able to - I’m using an Enterprise Name User (Node locked) Studio licenses with Enterprise Orchestrator but I am still able to connect the Studio to the Community Orchestrator without issue.
When you create the Robot in Orchestator, just make sure that you check the “Stand-alone license” so that it will be licensed through Studio and not via Orchestrator and not consume any licenses allocated in Orchestrator only. (If you look at the help screen in Studio it will indicate how Studio is obtaining it’s license)