If your WindowBlinds is a current subscription I believe you get a discount for upgrading from a stand alone program.  Check out the Object Desktop product page towards the botton https://www.stardock.com/products/odnt/purchase.asp .

No you don't have to uninstall the programs you all ready have and you shouldn't lose any skins that you downloaded.

When you purchase Object Desktop they will give you a serial number and a download link for a program called Impulse(if you don't already have it). Add the serial number in Impulse and it will detect WB,IP, and all your MyColors themes.

Impulse automatically registers your software listed in your Stardock.net account, there should be no need to 'enter' the serial numbers. When you first run Impulse, click on the 'My Desktop' Tab and you will see all your apps, IP, WB, even MyColors themes listed there. To update an older program, simply highlight it's Icon and use the 'Update' link to the right.