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 templates 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, Web links, News Feeds, Search) can be upgraded with a one click update. All third party extensions including templates 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 Joomla 4.x, you will be doing two 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 the best course of action.
Joomla 4.4.x to 5.x is an Upgrade, not a migration
Thankfully, Joomla 4.4.x to 5.x is an upgrade, not a migration. Head over to the Joomla 4.4.x to 5.x Planning and Upgrade Step by Step documentation.
Your server must meet the Technical Requirements of Joomla 3 or Joomla 4 to migrate on the same server.
Joomla core is the easy part. Third-party extensions including templates increase a migration's 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 or 2.5 to 3.10.x, you’re one up on the process. It will be much easier from 2.5 to 3.10.x and 3.10.x to 4 than from 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 few surprises.
Going from 4.4.x to 5.x is an upgrade. Once you get to Joomla 4.4.x, follow the instructions for [and upgrading to Joomla 5.x].