Normally, Win7 only accept upgrade from RC to RTM. It's sad but it's true. But
If there is a will, then there is a way
According to here, you can tweek Win7 installation file so it can accept upgrade even from Beta 1.
- Mount your Win7 DVD or Image
- Go to your sources folder
- Edit cversion.ini file
- Change minimum client (MinClient) property so it meet your system requirement. Basically, MinClient is Win7 build version. Change it into7000 if you have Win7 Beta version
- Save the file
- Run the installation as ussual
