Need help understanding what OS to buy.

Ok so i got the big screen for the 2500 with OS128. So i thought it should be just as simple as getting OS128xl and move on, but on the website it says the price of 128xl is $50 if the password of OS-XL is entered. This confuses the hell out of me because i though you couldnt use anything but 128 and 128xl on the big screen. Also if i just buy OS128XL does that mean im not getting the fully fledged OS because i never had OS-XL? I appreciate any help, thanks.

if you own osxl then 128xl is only $50 more as an upgrade. if you dont own osxl then 128xl is full price