How much is it to Upgrade From Pro 17 to 18

I had SketchUp Pro 17 and it expired. How much is it to Upgrade to 18 verses just buying a new license to Pro 18? Or do I not have a choice and I have to purchase a new license?

Go to the SketchUp site and check directly. License Manager | SketchUp Put in the serial number and your e-mail address.