You can install the 64-bit version using the same key that was used with the 32-bit install. You can not however upgrade from 32-bit to 64-bit. A clean install must be performed when changing bit versions.
My advice to you would be to purchase the computer with the bit version you want to use in the long run. If it is 64-bit that you really want then get a computer with the 64-bit OS already installed.
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