To trim a **Myrtillocactus Geometrizans Cristata** (the crested form) once it starts reverting to its regular columnar growth form:

1. Identify the Regular Growth:
- Regular growth is evident when the plant begins to lose its crest-like, fan-shaped structure and starts forming straight, columnar sections (as seen in your photo).

2. Prepare the Tools:
- Use a clean, sharp knife or pruning shears to avoid damaging the plant. Sterilize the blade with alcohol to prevent infections.

3. Choose the Cutting Spot:
- Trace the columnar section down to where it connects with the crested growth. Ensure you cut just below the reverting growth, preserving the crest's integrity.

4. Make the Cut:
- Cut cleanly through the base of the columnar section. Avoid jagged edges, as clean cuts heal faster and reduce the risk of infections.

5. Allow Healing:
- Let the cut area of the remaining plant and the removed section dry in a shaded area for a few days. This prevents rot by allowing the wound to callus over.

6. Replant or Propagate (Optional):
- The removed regular growth can be propagated. Allow the cutting to callus over for a week, then plant it in well-draining cactus soil. Keep it in a bright, shaded area and water sparingly until roots form.

7. Maintain the Crested Form:
- After removing the regular growth, monitor your plant for new shoots. Occasionally, reverting growth may reappear and need trimming.

By trimming the reverted growth, you help maintain the unique, sculptural appearance of your crested Myrtillocactus Geometrizans. In case of any questions, prior you start the trimming, please do not hesitate to contact me.