It doesn't cost anything to upgrade WeatherLink software for current owners of previous builds.
Just think there should be a small caveat here. It's been a (very) long time since WL had a major version number update, eg v4.x to v5.0. But when that last happened then IIRC the upgrade was chargeable and not free. It's difficult to know whether Davis are effectively going to allow WL to wither on the vine and maybe there never will be a v6. But if v6 should ever happen (eg something more like WDTU but with graphics and reporting tools) then don't be surprised if the update is chargeable again.