When a new version of OpenCart comes out, there are sometimes minor but tedious changes that need to be made to ensure my modules will work with the latest version. As you can imagine, this gets to be a lot of work for me to upgrade each module.
I have no plans to charge fees to update for small changes or point releases, as these are generally bug fixes or small changes, and will be made readily available in your account download page as soon as they are released. I always keep my modules up to date with the latest version within the first 2 weeks of a new release and 99% of the time they will be free of charge.
It is very unlikely, but In the case of major rewrites, I may request a small upgrade fee to cover the time, as a major rewrite would essentially be a completely new mod. This would be made public news if it were to ever happen. Again it is very unlikely.