I've decided to keep the booking. On looking more closely I remembered I'd booked a suite, at a very good rate and with breakfast included, because I knew I'd be dropping to Accor Gold and can no longer expect an upgrade. So the experience won't be very different and the handful of ALL points I'll miss out on won't significantly delay my return to Platinum.

I'll report back here if (A) I remember and (B) the thread has been moved to Hilton by then.