It's a custom built pc not an upgrade.Nothing on the new pc is connected to the oem license , therefore not valid.
You can't just build a new computer and grab a windows 7 oem license from say a dell,hp,gateway,etc. that is not running and use it,that is illegal.
If it's a non prebuilt oem license from say newegg , microsoft may work with you.If it's a branded oem , they won't allow it.