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OEM, Retail and volume licensing all use different product keys and cannot be inter changed.
If you have an OEM copy installed then the product key should be on the server. It looks like what you have bought two retail product keys.
To use the retail version you would have to reinstall as the easiest way to do it.
Since this is a technical forum and not a licensing one, if you need a definite answer you'd better contact your reseller or a trained Microsoft licensing specialist regarding your licensing questions.
Please contact the Microsoft Activation Center.