YAD7 is an open-source and freely available Drupal 7 compatible fork on Github. It has the same licensing and code as Drupal 7.
If you already have a Drupal 7 site, because YAD7 is an almost-direct Drupal 7 mirror, there is no real reason to convert to YAD7.
Nonetheless, if you wish to switch to YAD7, you can copy it directly over your existing Drupal 7 core. Copying YAD7 over your existing Drupal core is the same as updating your system to the most current version of Drupal. Therefore, all of the caveats involved with that apply. It is not a good idea to update to the most current Drupal 7 or YAD7 version from an older version, without first sequentially updating your site to each of the intermediary updates.
Here are some ways you can install YAD7 on your server:
- Git
- Clone YAD7 from Github
- Download
- Download a zip file from Github