Payment for webinar subscriptions.

The conditions for webinar payments were changed.  I should pay monthly for webinars. If I have only one webinar with attendee 3000 how can I change my billing monthly  subscriptions from 3000 to  500 attendee e.g. instead of paying 921€/monhtly to pay only 74€/month after my webinar with 3000 attendee. The  second question is - if I subscribe for monthly webinar and I immediately canceled this subsriptions may I use during one month for webinar with all functions?  Thanks.