Calculate forecast starting from the 2nd month?

We are using Oracle Fusion Demand Management and we run a demand plan on a monthly cycle.
Each time we re-run the plan, we need the forecast for the month that is about to start (the first month in the horizon) to remain unchanged compared to what was calculated in the previous month’s run.
In other words, when we run the plan this month, the forecast should only be recalculated starting from the second month onward, while keeping the immediate next month fixed.

Example:

• Plan run in March generated a forecast for April, May, June…
• Plan run in April