The best way to find out is to go to Sound Forge Pro's webpage - https://www.magix.com/us/music/sound-forge/ - select the version of Sound Forge Pro you want, and then go through the purchase process selecting the upgrade option. The process will ask you to enter an existing Sound Forge Pro serial number (14 in your case), and the purchase process will then advise you if you are eligible for upgrade pricing.
If you bought from Sweetwater, I would ask them why they are selling you an old upgrade when the new version has already been released. Or was it the offer in combination with Vegas Pro 18? This clearly says "Sound Forge Pro 14".
There has been numerous issues with purchasing from third-party sellers, (Best Buy, Musician's Friend, Guitar Center.. and now Sweetwater). I have always recommend purchasing directly from Magix. In your case, contact Sweetwater, as @Walter.I stated.