If you have already prepared returns using Premier (either Online or CD/Download), then there is no way to downgrade them, other then by starting over in the lower level product.
If you have TurboTax Premier CD/Download, yes you can buy Standard and use it if you need more returns. You can select Tools> Licence Code Switch from the top menu. This will allow you to enter a second code. Please make sure to save any open returns, as the program will restart.