But that link is for SME 6.01 and above 
The upgrade from 5.6 -> 6.0x is easy, all i did is put the 6.0x cd in the cdrom and booted the system and choose the upgrade and all went sweet. it should take you no more than about 10 mins to do a upgrade.
But when i done my upgrade i was not using any mods tho, and the server was running 5.6 for bit over a year.
HTH