Can I change my Windows 7 product key at a later date?

You can change product keys for like SKUs. If you're downloading the RETAIL (as opposed to Volume License) version of Windows 7 Home Premium from MSDN, and you have purchased the RETAIL version of Windows 7 Home Premium, then (usually) yes, you can change the product key. I say usually, because there have been some products that know they are the MSDN version and will not use a "regular" retail key. I doubt Windows 7 is one of these (I'm downloading it now as well, so I do not yet know).

Not sure how the E version will play a role, but I suspect it will not work as it is not a product you can upgrade to. Update: the E version has been cancelled, so this scenario is no longer an issue.

In any event, take backups. If it doesn't work, install the new copy you have purchased and restore your backups.

You can typically use the OEM tools to "reset" the activation and key of a computer and force the computer to ask you again on startup for the key. These OEM tools are available in the "OEM Preinstallation Kit" that you get when you buy Windows 7 in multipacks. Its probably available for download somewhere on Microsoft's website too.

I'm running the Ultimate version of the release candidate, and on the system properties dialog, at the bottom where it displays the activation status and product ID there is a link to change it. I don't know about Home versions, but I believe I recall seeing that link on my wife's laptop which was running vista home premium before I upgraded it.

you can use a simple yet very handy tool like Windows 7 Key Changer