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 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 3.x.
Joomla 2.5 to 3 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.
Your server must meet the technical requirements of Joomla 3 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, you’re one up on the process as it will be much easier from 2.5 to 3 than 1.5 to 3. If you are running 1.5 and going to 3, take heart, you skipped a migration in the middle. With proper planning you will have your new Joomla 3.x website up and running with minimal surprises.