Thanks for the reply Nic!
What I need to know is, if I buy cloud service, what happened to my old server license that I already purchased for certain amount, are their any way to convert that amount when buying cloud subscription or do I need to forgot about that server license cost and I have to pay full amount for cloud license?
Nothing happens. The server licence and cloud subscription are two totally separate things.
If your server licence was bought within the last 30 days, you can get a refund. If it is older than that, you can't. If it is two years old and you have not renewed, then it's completely irrelevant as it expired a year ago anyway.