Hi Greg,
If you are referring to the upgrade kit provided with the Free Windows 7 Upgrade Offer when buying a computer pre-loaded with Vista, you will always receive the same platform as that which was pre-installed. eg. if it came with 32-bit, the upgrade kit will always be 32-bit. Windows Anytime Upgrade works the same way.
The retail upgrade purchase will always come with both 32 and 64-bit versions, and the same product key will unlock both.
If your computer is pre-installed with OEM Windows 7 32-bit, then your key will also unlock 64-bit, provided that it is OEM, and that it is the same Edition (eg. Home Premium 32-bit will unlock Home Premium 64-bit). An OEM key will not unlock retail or Volume License media, and vice versa.
Furthermore, your manufacturer should be giving a choice before purchase of either 32-bit or 64-bit, and neither the OEM or MS are required (or willing, in most cases) to provide alternate media.
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There are 2 types of OEM media, system builder (generic/custom built) and hardware manufacturer (HP, Dell, Toshiba, etc.). If you purchased from a hardware manufacturer, then only media provided by the same manufacturer will activate properly. If you have a generic system builder license, then any generic 64-bit media will work.
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NOTE: The reason I put stars around the above paragraph: there are computer repair shops out there who will disagree with the above statement, and will be partially correct. Bottom Line: using media from a different hardware manufacturer is NOT supported by MS, and if any WGA errors occur as a result, MS will not help. This is the case even if reinstalling same bit media. This fact is documented by MS, but not widely known (even by MS employees).