|I suppose it's possible to go to Enterprise... but Why? It makes little sense and is VERY expensive.|
You would first have to buy the transition pack to remove the SBS restrictions and then you'd have to buy the Enterprise license and perform the upgrade. You cannot upgrade to datacenter since, unless I'm mistaken, it's only available in OEM editions - which cannot perform upgrades.
Just in case you were misinformed - SBS servers do permit other servers in the network - including datacenter and enterprise servers. They can be domain controllers, just not the FSMO master for the domain. So if you need a copy of Enterprise, buy a system and license and just add it to your SBS network.