Planning event, purchased 500 participant license but need to exchange for 1000. How?

Purchased the 500 large meeting upgrade, but now realize I will have more participants than estimated and need the 1000 license.  Can I somehow upgrade/exchange to the 1000 without forfeiting what I paid for the 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.