Large Meeting Add On question

Hello

I'm setting up an event and don't know how many participants will want to attend. It may be less then 100, but it may be more than that. If it's more than that, I would buy the Large Meeting Add On but I don't want to have to buy it if I don't need it.

Do I have to have the Large Meeting Add On already on my account before I schedule the meeting? Or can I schedule the meeting then buy the add on later if the number of attendees rises above 100 but keep the same meeting link? 

Thanks!