Yes, you can upgrade from Plus to Pro at any time.
We’ll reduce the amount you already paid for the Plus. So if, for example, you paid $39 for the Plus and then upgraded to Pro (which initially cost $99), the price would be $60 ($99 minus $39).
We change our pricing and promotions from time to time, which might make the Pro plan more expensive at a future point in time when you're about to upgrade. Because of that, if you think the Pro plan is more suitable for your use case (and in your budget), we’d suggest choosing it from the beginning to avoid extra fees and also be “worry-free” during your event.