Joomla tranform the code and increase features to compete in the open source way. Joomla version 1.0.x cannot be upgraded to Joomla version 1.5 stable version, in fact, it should be migrate not upgrade. Before to do that please note some very bold point in order to the migration became success, and very very successfull and get smile on your face.

References from,

http://docs.joomla.org/Migrating_from_1.0.x_to_1.5_Stable
http://forum.joomla.org/viewtopic.php?f=11&t=55065