A crossgrade is basically a special price for a user of a competitive product to entice you to add a new product to your toolset, in the hopes that you will prefer that new product to your older one, and become a customer of the new product in the future in preference to your old product. In the case of crossgrading to Dorico from Finale or Sibelius, you won’t have to give up your old product, and you will be able to both continue using and indeed purchasing further upgrades for them if you choose.
You will be able to put your Dorico license onto your existing USB-eLicenser and then run Dorico on your computers in exactly the same way you do with Cubase.