Through Whova's registration you can have a variety of tickets. You can easily create both paid and free ticket options for your registrants. If the ticket is free, you must select 'free ticket' when creating a new ticket. If the ticket cost money, you must select 'paid ticket' when creating a new ticket.
If at any point later, before or after the registration is published, you need to edit your tickets you can certainly make changes. Any changes will reflect immediately to the places your registration form is embedded.