It think @r2d2 brought up the key question. What is important to you?
A friend recently started adjusting his past calendar events so they were an accurate reflection of how he spent his time because he needed to justify the hours he billed to customers.
For me now, my calendar is only forward looking. I keep it an accurate representation of how I intend to spend my time so others can see when I am available. I delete cancelled events and move rescheduled events.
If you feel a need to keep track of what was cancelled, will you also need to know why it was cancelled and by whom?