Graduations usually happen later. But your study is completed on the date which is mentioned on the course completion letter that your educational institute issued you (one you used to apply for pgwp). If you don't have that then the date you got your transcript.
Even if your course completion is august, you can make these changes and then submit your application after actually competing your work experience (ie. August i assume). As long as your score stays above the minimum cut off in that draw and you have the experience you are claiming points for on the day you submit your application you should be perfectly fine. I strongly oppose declining the ITA especially if your score is low. You never know they might conduct all inclusive draw next. Saying this from experience. I had an ITA in May 2019 and declined it for somewhat similar reason then FST draw happened and score never came down. I had to wait a year to finally get an ITA again. Hope this helps!
