The easiest way to do this is to contact phone support and ask for the Hosting department. IIRC, you will have to purchase whatever level of VPS you want to move to and then migrate your content. I would call in though to make sure.
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I can only tell you my experience @globalsslng but I find that upgrading is a easier task than downgrading. You may be in a situation where you have to perform tasks yourself. Though my advice is to just use the current allocated resources by adding other websites to your hosting, you can downgrade. Call into support and they can walk you through what you will have to do. There is really different steps depending on the server you are on and other factors. I hope that helps?
...turns out that my two cents is worth less or more depending on the current exchange rate.
roy darling *my posts seem a lot shorter in my head