When running the Compare/ECO in PADS Layout try clicking on the Comparison tab and set the Name Comparison Options radio button to the 2nd selection where it says "Compare Net Names and Reference Designators. Prefer to Add ot Delete Parts instead of Renaming" and see if you get better results in the ECO file.
If alternate Name Comparison Options don't work, you may have to "seed" the changes. ECO does have problems with identical parts with parallel connections, decoupling caps being the biggest example, but as you've found, if the identical parts have enough of the same nets, you may not get the results you want. By "seeding", I'm talking about a partial manual ECO change to change the results of the ECO process in your favor. In this case, just copying U5 and labeling the new part U77, then running ECO Compare, may be all you need to get the results you want.
As a general note, never trust ECO forward and backward to be an automatic process. Always review the changes first, for if nothing else, it's just to make sure the changes are what you expect them to be.
Thanks, that did it, Mentor BIll.