iMac core duo to core 2 duo processor swap but no Lion

Ive just had my iMac processor upgraded to a core 2 Duo in order to install OS Lion but in App Store it wont let me install the new OS even though I now meet all the minimum requirements. Ive done a bit of research on the problem and have found it may be the serial number that still identifies the machine as core duo - is there a way to overcome this or is there a version of Lion that has been built to allow installation on an upgraded Mac ?

I only use the machine for development of iOS but the new XCode will not install onto Snow Leopard :(

Thanks for any help with this.