Dear Georgia,
You could start by applying scaling laws, e.g., as discuss on our forum here: 2.5 MW WT - #7 by Jason.Jonkman. However, you are not scaling the full wind turbine uniformly; instead, you are just shortening the tower, so, the scaling laws will only get you so far. You’ll likely need to redesign the tower. Again, aiming for a soft-stiff tower design with a first bending natural frequency between 1P and 3P would be a good design constraint.
Best regards,