Agree with torotil. This is not an upgrade process, it's an
acknowledgement that upgrading between major Drupal versions
isn't possible. Migrate is awesome but whether or not it's
in core, the key point is this: a Drupal site with a significant
number of custom and contrib modules/themes is going to take a lot of
time & money - relative to the initial design/build - to stay
inside the security coverage window.
More information about text formats