There is a cost related to the increase, but you also are prorated when you downgrade. As lost as you're not in session, you may increase the size of you upcoming events.
One other thing I'll mention, is that you can manually set the registration limits per event, so I would double check those through your My Webinars section online.
Yes, as long as you successfully upgrade your account before you start your webinar, the new attendee limit will be applied. The new limit will also be applied to all of your future scheduled webinars.
The only thing you cannot do is upgrade your attendee limit during a webinar.
As long as you have not already started the webinar itself, the new attendee limit will take affect. In other words you could have a webinar scheduled to start in 30 minutes time and if you upgraded your plan now, when you started that webinar in 30 minutes it would have the new attendee limit in effect.
Any plan changes to an active subscription -- like upgrading or downgrading mid-subscription period -- take effect immediately, while prorating any remaining balance forward to the next plan. All schedules, users and reports remain in place, while some features and attendee allowances are shifted on any remaining meetings.