one time meeting participants update.

Welcome to the Zoom Community, @NickCox.

There isn't really a "one-time" upgrade, but a "one month" upgrade is available.  Log in to the Zoom Web Portal with your account, then go to the Pricing page (https://zoom.us/pricing😞

Don't let the $600 per year text scare you!  Click on Buy Now (you're just browsing!) and see:

Make sure you click the Monthly (One Time) button on the 500 Participants section, with only 1 License indicated.  For $50, you will upgrade to the 500 Participants level for 1 month from the date you acquire the license.

Note that you will need to assign this license to your user login.  Let me know if you need assistance with that.