BPM Assist - Change One field then requires you to change another field or leave as is

Good Afternoon,
Need a little assistance. I created a Reason Code for PORel.DueDate changes so we can track why dates are being pushed back. I have a portion of it working using a Pre-Processing PO.Update:

I have it semi-working using a post process that checks to see if the new UD field reasoncode_c is blank or null. However, when I try to incorporate a warning or info pop-up to leave the reason as is, as there are instances where a supplier could be the same reason for multiple delays, I’m not getting the desired result. Can this be done with a simple Post Processing BPM or do I need custom code or a second BPM to fire off? Any advice would be appreciated.

Thanks in advance,