Planning for Migration
From Joomla! Documentation
Whether big or small, the first step in any migration process is planning. If planning takes a week or four weeks, then that is what it takes. Planning makes the migration process easier.
A few quick clarifying statements:
Joomla 1.5 to 3.10.x or 4.x is a major migration
The underlying structure of Joomla core extensions changed significantly. All data including core content, third-party extensions and template needs to migrate from 1.5 to 4.x.
Joomla 2.5 to 3.10.x and 3.10.x to Joomla 4.x is a minor migration (or a mini-migration)
A mini-migration means that the Joomla core extensions (Articles, Menus, Modules, Categories, Contacts, Weblinks, Newsfeeds, Search) can be upgraded with a one click update. All third party extensions including template need to be addressed individually.
Joomla 2.5 to 4.x is two minor migrations (or two mini-migrations)
If you are currently running Joomla 2.5 and want to migrate to Joomla4.x, you will be doing two (2) mini-migrations. The first will be from 2.5 to 3.10.x. The second will be 3.10.x to 4.x. See Planning for Mini-Migration - Joomla 2.5 to 4.x for the links and best course of action.
Your server must meet the Technical Requirements of Joomla 3 and/or Joomla 4 to migrate on the same server.
Joomla core is the easy part. Third-party extensions including templates are what makes a migration increase in the level of difficulty.
Final Thoughts and Considerations
There are many considerations for both major migrations and minor migrations. If you’ve already done a migration from 1.5 to 2.5, and/or 2.5 to 3.10.x you’re one up on the process as it will be much easier from 2.5 to 3.10.x and 3.10.x to 4 than 1.5 to 3 or 4. If you are running 1.5 and going to 4, take heart, you skipped two migrations in the middle. With proper planning you will have your new Joomla 4.x website up and running with minimal surprises.