After a while the SQL Express database limit kicks in and an SQL upgrade is inevitable.
Luckly the developers at microsoft also have encountered that special breed of managers and they have provided us a simple setup parameter to upgrade your SQL Express to your new SQL SKU (Standard or Enterprise).
Take your offical MS SQL Server CD. Open a command window. And start the setup with the upgrade parameter.
Ofcourse this can be used for every SQL Express setup. You don’t need to have SharePoint installed to make use of this parameter.
That’s all there is to it.