It should just be a normal upgrade. Except that there is an extra license file to intall. So back up everything, apply the new version of code, add the license and away you go.
Anyway if you choose to upgrade to Alfresco Enterprise then I believe that Alfresco Support is on hand to help with the migration.
And also hold back on upgrading to the latest Community version (as for now 3.4.a) if you know you want to go to Enterprise. Community version is ahead of Enterprise in terms of newfeatures (and schema version number), and it takes a couple of months until the corresponding Enterprise version is avaiable.