Windows 7 - migrate to 64-bit

My t400 comes with pre-installed Windows 7 32-bit.
Is it legal to install Windows 7 64-bit on them, and what serial number to use?

 

Just use the serial that came with your laptop. Technically you're not allowed to upgrade from 32-bit to 64-bit using an OEM key, but Microsoft doesn't really seem to care.

 

It's legal to use a OEM DVD image of Windows 7 64-bit with the serial key on your t400. It's apparently illegal to use a retail image of Windows 7 64-bit, but you can do it too. In either case at most you'll just need to call in to Microsoft to activate it (it'll take 5 mins).

 

From my understanding, you should be able to use the same serial key when migrating from 32 bit to a 64 bit OS. Though you have to make sure that you're installing the same version as the one you used before (i.e. Professional to Professional x64) and use the same hardware. Bear in mind as aznguyphan said, you have to use an OEM disc to install. A standard retail or VLK disc of Windows 7 will not accept OEM keys.

I'm fortunate to be a MSDN member and have access to retail keys of Windows 7, regardless of whether it is 32 bit or 64 bit, the keys are interchangeable to both so I don't think it should be a problem for you. Out of experience I have installed many OS' such as Windows Server 2008 from 32-64-32 bit with no problems using the same key.

 

according to Lenovo you can order a 64 bit OS from them for the charge of service and handling fee (the cost of getting a new lenovo recovery media).