SP Upgrade is able to migrate (convert and transfer) the core data from Joomla! 1.5 (from now on called source) to Joomla! 1.6, 1.7, 2.5 or 3.x (from now on called destination) and above. You install SP Upgrade on the destination site, and you transfer items from the source database.
In no case an item will overwrite an existing item with the same id during transfer if you choose it. There is also a restriction in Joomla! where you cannot have the same alias of an item under the same parent. Taking these facts into consideration there are two important options you have to decide before the transfer.
If there is duplicate id, whether:
- to transfer the item with new id, (recommended for already existing destination site with extensions, or other data installed)
- replace existing items (recommended for new empty destination sites)
- or not transfer at all.
The component keep history of what is already transferred. This is necessary to keep track in case you have a disruption of the process for any reason (timeout, memory, network)
It is strongly recommended to backup your database before proceed with the transfer, just to be on the safe side.
Another very good and more detailed guide you can find here: http://www.ostraining.com/blog/joomla/sp-upgrade/