To better keep track of who attended your event live vs. on-demand, create a separate ticket for On-Demand Only attendees.
Go to Registration > Tickets > Create New Ticket
Make sure you set the sale start and end date for your tickets for as far out as you need. You can do this under Create New Ticket > Advance Settings.
We recommend setting the "Ticket sales end date" for as long as you scheduled Replay to be available.
Don't forget to close your other ticket types to avoid confusion. You can do so by changing the "Ticket sales end date" under Tickets > edit ticket > Advanced settings.