Are you talking about creating a FDGB and then moving into an RDBMS, or are you talking about "converting" an Oracle GDB to a SQL Server GDB (or vice versa)?
ArcGIS Diagrammer is a free tool that allows you to craft your geodatabase model in an environment agnostic to a specific RDBMS. I've had good luck with this tool from time to time from a basic design standpoint where I can create tables, feature classes, rasters, relationship classes, and other Esri-related objects. It's a handy tool for quickly modifying the order of fields and their names, too. Once you save your project file, you'll be able to export a schema XML document which can then be imported into an enterprise geodatabase in SQL Server or Oracle (or a FGDB if you chose). This really isn't "converting" the database from SQL Server to Oracle or vice versa (that is, keeping your data intact), but it does give you flexibility to manufacture a data model and load its empty schema wherever you desire.
That being said, if you need a way to move your data from one RDBMS to another (which may not be what you're trying to do, I can't be sure), you may be able to simply use a manual copy and paste the objects from one geodatabase (let's say, Oracle) into another geodatabase (SQL Server) if you establish connections to both in ArcCatalog. (Note: The Copy Data tool won't work in the case where relationship classes are involved whereby one object class is related to more than one other object class.) You undoubtedly will have to work through granting your permissions on the target afterward, though.
