You may freely upgrade your version of BirdieSync for minor updates but you you may have to pay an upgrade license fee for major updates.
Users owning a license for BirdieSync 2.0 can still use their license with BirdieSync 2.4. No need to upgrade.
If you have a BirdieSync 2.x license and wish to use BirdieSync 3.0, it is necessary to upgrade your license.