Let's say you are filling out a form. When you are done you can say, "I have completed this form."
It is in a state of completion because all of the required entries are filled out.
Now, let's say you need to make a few revisions, so the very first thing you do is to erase a field on the form. You've now brought this form to a state of "incompletion"
What verb adequately describes this notion of "to make incomplete?"
e.g. "When I erased this field I have ______ this form." (past tense example)
Something like this? "I have uncompleted this form."