Modification of sales and purchases header

I come back to the question of modifying sales or purchases headers (an issue is open: Issue 9487: Currency not set correctly when modify header - Tryton issue tracker).

I think it is not possible to consider as a feature something which modifies the data already entered (or reset) by the user, am I the only one who finds that annoying?

How to make so that the ‘on_change’ are not called in this case? a suggestion ?