While every situation is different with specific combinations of components, modules and plugins being used — and understandably different client concerns and needs — there are some basic guidelines that you can use to help you make the best decision for your site.
To be clear, as of this writing 1.6 is no longer an option. That leaves 1.7 or the upcoming 2.5 version of Joomla!, which was renamed from 1.8 to 2.5 — voted on by the Joomla! community — to clarify that it is a major, long-term release.
Joomla! 2.5 is not really an option at this time since no stable version is currently available, but it is on the horizon and should factor in your plan during the coming year. The due date for the release of Joomla! 2.5 GA (general availability) is now January 24th. It might be some time before compatible extensions for 2.5 are available.
Why Migrate Away From Joomla! 1.5? Answer: Security Updates
A good question. Your site runs great on 1.5 and the software is stable. Why mess up a good thing? In addition to a host of cool new features in 2.5, the main reasons why have to do with future security and support. Joomla! 1.5 reaches its end of life in April of 2012, which means that after April the Joomla! project team will no longer provide security patches if any security vulnerabilities are discovered. With the rise of automated probes by "script kiddies" http://en.wikipedia.org/wiki/Script_kiddie — low-level "crackers" running downloadable attack programs — looking for outdated and insecure extensions and/or more advanced spammers looking for ways to break in and harvest email addresses from your site or server, keeping up with patches for Joomla! and for all third-party extensions becomes very important.
And as happened after the end of life of Joomla 1.0, third-party extension developers will probably begin to drop security support of their 1.5 version extensions.
Both of these are good reasons to migrate since vulnerabilities of any kind in either the core Joomla! code, or extensions can provide "crackers" with a means of gaining control of your server and wreaking havoc and/or obtaining sensitive data.
Do I Migrate or Rebuild?
This is a big question and will take a fair amount of research and thought. While there is a tool provided to migrate the core Joomla! site from 1.5 to the newest version of Joomla!, it is likely that no such migration tool will exist for your extensions and their data so you may have to rebuild those features.
Familiarize yourself with the core migration process to the point where you can accurately estimate your time to migrate. Then list all of your installed extensions, verify the existence of updated versions and find out if there are migration tools available for each. Then evaluate the size and structure of your site. If you determine that you have little content but lots of extensions to configure, you may decide to skip the Joomla migration and rebuild the core of your site and then install and configure all of your extensions. If you have a lot of content then migrating may make more sense, since the Joomla! migration tool will migrate all of your Joomla! content articles for you.
Note that sites running on the first generation Joomla! 1.0 CMS will still have to be migrated to Joomla! version 1.5 first, and then migrated again to the latest version. (Building from scratch may be a good option to consider.)
Guidelines for Determining When to Migrate:
Are The Extensions You Need Available?
Perhaps the number one consideration is whether the extensions you are currently using are available in stable versions for the version of Joomla! you wish to migrate to. You'll need to be able to upgrade those so that you'll have your current functionality in the newly migrated site.
Hopefully, when originally reviewing potential extensions for your site, you did enough due diligence to determine that the developers of those extensions were committed to continue development of their extensions for the long term, and that they have indeed continued to provide updated versions of their extensions in a timely manner for the new versions of Joomla!. If so, well done, you're one of the lucky ones. Fact is that with so many extensions available, it's definitely possible that you may have to find a replacement extension to use instead of one no longer supported by the developer.
When evaluating an extension to use, I make sure to read the reviews of the extension on the Joomla! Extension Directory (JED) to see if people like the extension, had problems with it, and whether the developer responded to problems with replies. I also visit developer sites, check out their demos and look to the extension developer's support forums for signs of continuous activity by users and support staff as a sign that the developer is likely to continue to provide security patches.
Template Concerns During Migrations
If you are utilizing a custom template in your 1.5 site, you should be aware that Joomla's core HTML output changed significantly from Joomla! 1.5 to 1.6, and that CSS class names and IDs have changed, which will affect display of content in your site. There have been other changes in how template parameters are defined, the additiona of a new 1.6 Doctype, and more which you can read about in the official Joomla docs.
If you purchased a template from one of many template clubs check to see if your template has been upgraded to be compatible with 1.7 or 2.5. I wouldn't expect the template clubs to start offering 2.5 versions until a stable version of 2.5 comes out. But it may be that 1.7 templates may run on Joomla! 2.5, since the Joomla! core team is saying that the update from 1.7 to 2.5 will be "seamless". While you're at it, you might discuss a site re-design with your client. It would be a great time to redesign to give the site a new look while you're in there.
Relevant information regarding this topic:
- Migrating from Joomla 1.5 to Joomla 1.6+...
- JUpgrade Migration Tool...
- Nine Questions When Preparing Clients for Joomla 2.5...
- Preparing for Joomla! 2.5 — Coming January 2012...
- Upgrading a Joomla 1.5 Template to 1.6...
- Upgrading and Migrating Joomla...
Tidbits for Joomla! 1.0 Migrations: