Hello, I have had GSX for a while now and have never updated it.
I strongly suggest downloading the full GSX installer now and install it.
So in FSX's drop down menu I went to add-on manager and it asked if I wanted to update it. I said yes. So it gave me a website. I downloaded the file called setup_addonmanagerX.exe.
That's the Stand-Alone Addon Manager, which represent a "Major update" request, which is normal in your case, since you said you never updated.
However, the GSX manual explains that, even a major update, will only update the code. Vehicles, sounds, textures, liveries (which surely you are missing now, if you never update), can only obtained with the full GSX installer.
That's why, if you never updated, it's best if you ignore the Stand-Alone Addon Manager, and download and install the full GSX installer.
Will it know I have the paid version on update?
Of course it will.