Asking for the favour twice, politely
The wording that keeps a postponed room full is a straightforward apology plus a clear out. Apologise once for the change, not three times, and give people an easy way to say the new date does not work. "Reply if the Tuesday does not suit and we will hold your place for the following run" costs nothing and converts a silent no-show into a name you know.
Then send one more note four or five days before the new date, even if the reminders are covering it. A postponed session is a weaker memory than an original one, and a short line saying "still on for the twenty-third, same hour, your access link is in your first email" is worth more than the length you think it needs.