Several times a year, we provide minor version updates that can be downloaded by going to Help > Check for Updates (non-App Store version). Minor version updates for the App Store version are provided automatically as part of the built-in OS X software update.
These “Minor version” updates -- e.g., version 3.0 to 3.1 -- are available at no charge.
“Major version” updates -- e.g., version 3.0 to 4.0 -- require the purchase of a new license. Previous customers may be eligible for a discount depending on the original date and version of purchase.