Can I upgrade meeting capacity while a meeting is in progress?

Welcome to the Zoom Community, @adver.

Normally I like to be able to point out the source of my answers, either as Zoom Support articles, or the results of my own testing. In this case, however, I’m relating an historical experience to which I was only an outside observer. I can’t say my impression of what happened is indeed factual, nor can I say that it remains to be true today.

In fall of 2020 I was attending an annual meeting of a large organization. Hosting and Zoom tech support was provided by an outside company. As is often the case, I believe that the vendor assumed that there would not be full attendance by everyone at any one session, and I believe they had a Webinar 500 License.

At one Webinar, however, the authors of an important book on parliamentary procedure were scheduled to discuss the imminent release of a new edition. As the number of attendees grew to 500, an announcement was made that there would be a delay while they arranged for an “overflow” livestream to YouTube. More delay ensued, and it was announced that there was some technical issue with getting the livestream started. About 5 minutes later, the attendance figures began to increase, ultimately settling in the 800 range!

Based in this experience, it my impression that it is possible to upgrade your capacity after the session (Webinar or Meeting) has started.  My original (and still often-stated) understanding is that Zoom checks the session capacity as it opens the session – meaning that you can register/invite as many people as you like in advance, and you can increase the capacity any time before the meeting starts – even “just before”.  However, I do believe that it might be possible to upgrade the capacity during the session if necessary. 

But I wouldn’t count on it. Not if my reputation depended on it!  You have to decide for yourself if the risk that it might not work this way will impact your session. If it’s a paid session, build your pricing model to allow for the extra license if even 1 more person needs to attend.

If it’s an open access session, consider the livestream approach for over-capacity attendance. 

Welcome to the Zoom Community, @DrEseza.

Most of Zoom's pricing information is located here: 

https://zoom.us/pricing

Look for the Add-Ons section for the Large Meetings. Once you click the Buy Now button, you'll see that you can upgrade to 500 or 1,000 attendees.  If you do not need this on a long-term basis, be sure to pick the "Monthly (One Time)" payment option; the fee for a 1-month 500 attendee Large Meeting add-on is $50 USD.