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.
Read how to migrate your old 1.5 site to the latest Joomla! release, and answers to frequently asked questions.
History Log assist you identify individual items, and whether they were transferred. It is also a necessary part of the copy process, so in case you face with any interruption (e.g. due to timeout, memory, network) it can keep track and when you rerun continue from where you had the timeout. It is very important not to delete the history log, until all items are Transferred and Finalized.
Find here answers to the most common questions you might have about SP Upgrade and it's usage.