Its been quite sometime since Joomla has releases latest version of the application(3.x), however many of the users are still stuck on old Joomla versions such as 1.5, 1.6 and 1.7
Some of the users are on Joomla 2.5 as well but the number is relatively less as Joomla 2.5 and 3.x were released almost at the same time. Also, migration from joomla 2.5 to 3.x is relatively straight forward. However, migration from 1.5+ series is not so straight forward to say the least. It involves many steps but primarily it is:
 

1. Migrate core date
2. Identify the third party components, plugins and modules and migrate the same
3. Migrate or create a new template.

Chances are some of the 3rd party components you are planning on using may not be supported on the new version. In that case you only have 2 options. Either to hire a developer to migrate the plugin to new version or migrate data to a new component on latest version.

CodeFire has carried out many migrations from old joomla versions to latest one, if you need any help feel free to contact us