Hello @Bridget111,
If you mean that you used the pay less upfront option to pay 50% on making the reservation and 50% later then some reports here say that the system will try to charge the remainder to the original card and ask you to separately give the details of the new card when the original request fails.
You could wait to see if that happens or contact Airbnb support in advance.
There's no point in contacting the hosts as they don't have your money. Airbnb charges you and pays the hosts only after you check in.
For information on ways to contact Airbnb support type "contact airbnb" in the "Search the community" box on these forum pages where you posted your question. Scroll down a little in the list of suggestions that appears and click on the "Contact Airbnb: A Community Help Guide[UPDATED]" one. That lists phone numbers and ways to contact them via Twitter or Facebook.
Steve.