Do I have to pay full price to upgrade?

No. If you have bought it already then all future updates are included for free (Royalty Free). To get the new version of the addon go to the store you made your purchase (either BlenderMarket or Gumroad) and grab the latest version.

I use Cycles and see fireflies (strange white artifacts)

Fireflies are abnormally bright pixels relative to their surroundings. Unfortunately using Cycles in combination with scenes that have low light levels will cause fireflies. This effect can be avoided by increasing samples (Render properties > Sampling) or using Super Image Denoiser. For more detailed information see this issue.