How To Migrate CiviCRM From Wordpress To Drupal?
Assuming that a) you gave CiviCRM a separate database under Wordpress, and b) you installed the same version of CiviCRM under Drupal:


Dump the current CiviCRM database to a .sql file.
Load that database into Drupal.
Review "Moving an Existing Installation to a New Server or Location" for relevant details regarding deleting caches etc.
Delete all the entries out of the civicrm_uf_match table in the CiviCRM database (while logged in as user 1).
Go to Administer menu > Users and Permissions > Synchronize Users to Contacts.


That should repopulate your civicrm_uf_match table and thus connect your Drupal users to contacts.  Log out and log in again (in a different browser) to make sure everything looks good - and clear your caches again!