...You need to not worry about the ideal_flight.PLN since that is married with the scenario flight setup, instead simply export another plan file but with the ICAOICAO name desired.
In the Export window (button on Flight Gen page), make an Export object by choosing the FSX .PLN file type in the drop down list at the top. Name it ICAOICAO.PLN in the File Name box. Have it export to your required folder with the Add Location button. If you already have a flight built, then you can use the Export test button lower left to see the result right away.
I hope that helps!