Migrating Joomla Versions
Alternative method
There is an alternative method you can use on localhost or on your server. Third party extensions that are compatible with Joomla 2.5 and the latest Joomla version often use different files depending on which version of Joomla they are installed in. This method prevents prevents the third party 2.5 files from interfering with the update using the files from a fresh 2.5.28 site to update your site's database.
Make sure you have a backup and your site is up to date https://github.com/joomla/joomla-cms/releases/tag/2.5.28 , the 'remember me' plugin is disabled.
Stage #1
- Delete the Joomla files.
- Install Joomla 2.5.28 to an empty database.
- Edit the configuration.php to connect to the original database (the one with your site data is in).
- Update to the latest Joomla version. (you will berequired to log back in a few times).
Your original database (the one with your site data is in) is now up to date.
Stage 2#
- Delete the Joomla files.
- Install the latest Joomla version to another empty database.
- Install the 3rd party extensions (that are compatible with the latest Joomla version).
- Edit the configuration.php to connect to the original database (the one with your site data is in).
You now have the latest Joomla files and 3rd party extension files (that are compatible with the latest Joomla version) connected to an updated Joomla database which contains your original data. All you need to do now is replace your image folders/files and other custom files such as 'Template Overrides'.
N.B.
Uninstalling extensions should also remove their tables from the database. But some extensions (when uninstalled) leave their tables. You can remove those tables manually if you wish. Or if you use Akeeba backup you use it's settings to deselect the tables from being backed up.