Licenses are tied to your Synology Account, not the hardware. You can deactivate a license from an old NAS and reactivate it on a new one. This is not "free" but it is perpetual.
: Most standard IP cameras use 1 license, but complex devices like multi-lens cameras or video servers may require multiple licenses per physical unit. to pool your existing NAS licenses? Surveillance Device License Pack | Synology Inc.