Hi Matrix,
It is not difficult as long as you follow correct upgrade procedures,
If the database is local, (you will need management studio to work on this),
1. Take backup of your current vSphere 5.0 Database.
2. Restore the backup on the new Physical server where you will install the new 5.5 vCenter.
3. Create new ODBC connections, and point to that Database.
3. Install the vCenter 5.5 and point to the above ODBC during installation.
If the Database is not local and is residing on some other server.
1. Still take the backup of the DB instances.
2. Create new ODBC connections, and point to that Database.
3. Install the new vCenter 5.5 and point to that ODBC.
thanks,