Completed activities shows Completed By another user when owner of account changed.

Question

one of our customer reported that CRM shows phone calls completed by her when she did not complete the phone call. On further investigation, it was found that the following triggered the update.
1. User X changed the owner of Account to another user - user Y.
2. Since there is a parental relation between Account and Activities, all the activity owners where changed into user Y.
3. On all the active and inactive activities, the modified by was changed to user X.
But as a side effect on the Posts/Activities/notes control, all the modified activities now shows as completed by user X. - These activities were completed by some other user a long time ago.
Is there some way to change the "Completed By" to show "Modified By" instead? Attaching a picture to clarify.
Regards,
Jithesh 

Answer 1

Activities Wall don't support customizations to change these label.
I think in Activities there is no field to capture Completed By. It only captures Modified By. And updates it with User which modified it last.
Hence you will not be able to retrieve who actually completed it. Usually Completed activities can't be modified that's why it shows last modified user as Completed By.