How not require re_approval of Purchasing Order after change Promissed Date (PROMISED_DATE) of Lines

Hi everybody,

We need to apply a customization when the promissed_date of Purchasing  Lines was modified not requiring re_approval, how example having the same actions of TRANSACTION_REASON. When I change the TRANSACTION_REASON the system not require re_approve. How apply it on PROMISED_DATE too ?

I created an customization of workflow "PO Approval Top Process" but I saw that system call this workflow only I clicked in Approve button and I need to apply this action when I modify Promisse Date and click on Save Button as the same process to TRANSACTION_REASON. 

Somebody know how do this ?