Internal Users cannot be moved to another Organization. The recommended workaround is to delete the User and create the User again under the new Organization. Deleting a User will allow you to reassign any associated Leads, Deals, MDF Requests etc.
Partner Users can be moved to different Organizations. Select the User that you want to move to a different Organization and click the Actions button, then select Move.
Upon moving a Partner User to a new Partner Organization, you will be prompted to reassign any Deals, Leads and MDF Requests (that are assigned to the User) to another User within their current Partner Organization.