Unable to activate Win7 after re-installing from 32-bit to 64-bit

This actually isn't my own problem, but someone I'm trying to help. Recently they re-installed Windows 7 64-bit Ultimate from an Ultimate 32-bit installation, both the same key/copy. After installing 64-bit Ultimate, Microsoft says their CD key is blocked and are unable to activate. They attempted to revert to 32-bit from frustration, but now 32-bit is blocked, as well. So both 32 and 64 are blocked.

For years they used 32-bit. During those years, they changed their motherboard and hard drive. They used the 32-bit version on both motherboards and different hard drives without error. Their license is a retail version, not an OEM. But, is it possible Microsoft has the CD key tagged to the old 32-bit version and sees the new 64-bit installation as invalid? In other words, is this potentially an error on Microsoft's end?

Addendum: We know their license is retail because their initial purchase of Win7 Ultimate came with a builder's kit key. After a year passed, they were able to obtain a legitimate retail license from Microsoft after explaining and proving their situation with the initial vendor.