Update Line from Header automatically?

Hi guys.

I have a table Header (prim.key=No.) and Line (prim.key=Line No., Header No.) where Header No. is No. from header table because Header Card is connected to Lines listpart using this field.

In my Line table, in OnInsert() i have the code which makes sure i get the date from Header on Line:
IF Header.GET("Header No.") THEN
Date := Header.Date;

It works fine when i first enter the Date in Header and insert something on Line.
But when I wish to change the date on Header it doesn't get changed on Line.
What should I do?

I want, when i change the Date on Header Card, for Nav to check if there are existing lines for that header, and if there are, to ask if I want to change the date. If I confirm, date gets changed on Line. If not, it doesn't.

I really appreciate your help
Thanks