Upgrading from Joomla 1.5 to 1.7 requires "migration".
The Joomla 1.5 Template may need editing, or a new Template for J1.7 might be available at your Template designer's site.
This tutorial explains how to convert a Joomla 1.5 template to 1.6+.
It is possible to migrate directly from 1.5 to 1.7 bypassing version 1.6.
jUpgrade will migrate Joomla directly from 1.5 to 1.7.
This tutorial at joomla.org explains how to migrate from 1.5 to 1.6 and up.
