We don't yet know when in March early access to the game will be released. In my experience, Frontier tend to stay quiet on release dates until they're sure they can meet their deadline, so we'll probably get something in the region of a couple of weeks notice.
I would expect the early access phase to be somewhat similar to their other game, Elite: Dangerous, where they added features in milestone builds through the early access period, with smaller but more regular bugfix updates. I'd expect Frontier to be updating the game right up until launch, after which you'll be able to play the release version (i.e. if you have the early bird edition you don't need to buy the game again). I don't know if the Steam version will be available at launch, they added it later in Elite due to popular demand. It wouldn't surprise me if it's available for Planet Coaster from release.