We actually tried that as well but it did not work.
I did find a potential cause for the issue though. The Client uses a practice of having several individuals who creates everyone else's calendar events. These individuals essentially act as schedulers. Because of this the users being scheduled do not actually show as the owner of the events at all, the scheduler who created the event for them does. So when I use the feature to transfer ownership of all the events in the users calendar it finds they don't have any and transfers nothing.
Additionally some of these features only transfer future events and they needed to preserve the older events as they act as records and contain notes.
I was able to find a work around (with the help of Google Support) by exporting the individuals calendar events entirely as an .ics file. Then I can easily create a new calendar naming it "{deleted user name} TRF" and importing the .ics containing all of the users events the schedulers created for them. This preserves all of the events notes and gets everything instead of just future events.
I can then easily give access to the new transferred events calendar to the users who need to have access to them. The entire process only takes me a few minutes since GAM makes giving ACLs permissions and adding calendars for multiple users simultaneously pretty easy. I can do that part with two commands which I have preformatted with all the schedulers included.