Do you have to upgrade to Joomla! 1.5?
From Joomla! Documentation
The short answer is no.
The longer answer is if you have an existing site running version(v).1.0, you do not need to upgrade. That said v.1.0 is no longer supported for bugs and security, and is certainly not receiving any new development. Therefore, users of v.1.0 will not benefit from improvements in performance and security arising in PHP, MySQL, and other supporting software that Joomla utilises.
Current development status
All new core development work is concentrating on v.1.6. Version 1.5 continues to be supported for bugs and security fixes, other than this it is considered stable and therefore unlikely to have any major changes added to it. Also, all development work on new Extensions concentrates on v.1.5 and only this version is currently supported by the Joomla! Extensions Directory (JED).
Recommendation
The recommendation must therefore be that you upgrade any sites that are running v.1.0 to v.1.5 as soon as possible. There will be no official migration or upgrade path from v.1.0 directly to v.1.6 - when it is released - so it would be better to make the changeover to v.1.5 sooner rather than later.
If you are new to Joomla or an existing user starting a new site then you should only consider working with v.1.5.
The future
It is also worth adding that, at the time of writing, v.1.6 remains in Beta testing (release 14) and must not be used for live sites, although its use and testing on secure local and private remote servers is encouraged to assist developers iron out the last few kinks and wrinkles.
The Joomla Bug Squad add an FAQ page for each release that collates known issues and proposed solutions that arise with each release. If you are having a problem with a current release, check that page for possible solutions, or add the issue you are finding.
Further assistance
Continue to post requests for assistance in the forums.
More information about v.1.5 can be found in the Version 1.5 FAQ pages and by searching the forums.
