How to shift a MVO curve down and to the left?

How is it possible shift a MVO curve down and to the left? because MVO curve is concave down, increasing, shift it down and left means you arrive at the original position.

Shouldn’t the introduction simple move alone on the MVO curve instead of shift it?

Reduce both risk and return.

You are confusing shifting the entire curve (that is, physically moving the whole curve - scenario #1) to a movement along the curve (that is, using one and the same curve - just choosing a different point along it - scenario #2). Scenario #2 can happen due to the introduction and removal of asset classes identified through the MVO process into your portfolio. Scenario #1 (which is what we are concerned with) is due to something outside of your portfolio. Taxes is an issue outside of your portfolio, and will change your risk and return characteristics - as pointed above - even when you do not change your asset base.