Here is the question I sent to Zoom Support:

I have Zoom sessions, and am planning a one-session event for the end of February. I have the monthly-payment plan (100 attendees max), because my next event won't happen until next November, and I will cancel my Zoom Sessions in March, and reinstate it in October. My February meeting is expected to have about 200 attendees - is it possible to add the "Large Meeting" add-on to "Zoom Sessions" specifically for my Feb event?

Their response did not actually offer a reasonable solution:

"I understand your concerns regarding the upgrade of your Zoom Sessions participants by purchasing the Large Meeting add-on. I'll be glad to assist you with this.

Please be informed the Zoom Events and the Large Meeting add-on are different plans. In order for you to increase your Zoom Sessions' participant capacity, you need to upgrade from Zoom Sessions 100 to Zoom Sessions 500."

I replied with the following:

I do not want to pay $4500 to have 250 people in one session.  This is a one-off session that likely will never happen again.  Typically any of our sessions are for less than 100 people. I do not need an annual subscription.

Is Zoom sessions 500 available on a monthly plan?  Does not seem to be... but can it be for me given my situation? What is the cost to stay on a monthly 100 sessions plan if there are additional people above 100 that join?

I am hoping that a literate human with actual hands-on experience would have a solution for me. I am willing to pay per person for attendees over 100, but no one will tell me how much that costs. I have unsuccessfully attempted to get answers through their chatbot, and then I believe the customer support module was also AI. No one seems to understand my request.  I don't want to switch to "Events" or "Webinars" but maybe I have to?  I like the production value features of sessions... 

Thank you for your advice!