It's possible to upgrade a Joomla 1.5 installation to a 2.5 or 3.0 version of Joomla. After doing this you will find that the database you end up with it totally different to the usual 2.5 installation. They will contain a mixture of 2.5 and 1.5 tables.
The jupgraded 2.5 database also seems to contain other differences when compared with a standard Joomla 2.5 installation. This means that it's not possible to port one jupgrade "database" from one Joomla installation to another. You can move any Joomla site but moving just the database breaks the install.