This will vary from site to site, but migration is basically a complete rebuild. At minimum, your website template will need to be converted to a Joomla 3.x compatible format, and all content and menu structures moved over. This is a small to medium task. An audit of third party extensions currently in use on the site will need to be carried out, and suitable Joomla 3.x compatible replacements found. The extent of work required will vary greatly depending on how complex this task is, and this can only be assessed after a site audit.
Given the amount of work required, many customers might seriously consider completely redeveloping their website from scratch. If you've been considering giving your website a facelift, or introducing new functionality, now might be a good time to start finalising the project requirements.
