I want to upgrade 500 Participants to 1000 Participants

I purchased the 500 Large Meetings upgrade, but now realize I will have more participants than I anticipated and will need 1000 licenses. Can I upgrade/exchange to 1000 without forfeiting what I paid for 500? Or is the $50 I spent on the 500 wasted? I can't find a way to contact a person in billing to help with this question.

@Openai-Alan , Upgrade Purchased