Hello Lisa,
Please refer below related to Workflow rule (Not Mass Change).
Can you try creating WorkFlow Rule for 1)Business Object "Account" with 'Timing' as "On Create Only" 2)Define Condition : Status 'Equal to' as "In Preparation" and 3)Define Action: Select a Rule Type as "Action" and 'Select Action' as "Activate".
Please check if it works and solve your concern .
Regards,
Ankush.