Tommaso, yes you can. When you run the v26 installer it will check for an installation of v25. If it sees this installed, you will be given the option to remove this version or install v26 alongside it. Make sure that you choose No for this option as the default setting will remove it.
The reason that Adrian was only seeing the $200 trade-up option appears to be account-specific.
Mike, it sounds like you're referring the product description of a Finale upgrade as it appears on the reseller's site, is that correct? If so, then that would not be related to the specific behavior Adrian is seeing on the MakeMusic store page.
I am disappointed that V26 is only available as a download. I understand that this will help with the company but such a large download is very difficult without decent broadband. Perhaps the smaller V26 program could be a download with the Garritan libraries provided on a usb if required.