Hello @Marin33,
What should happen is that Airbnb will try to get the payment from the old card, it will be denied and you will be asked to pay with a different method.
There is no "Pay second half now" option. Airbnb completely controls the payment times, guests use the site to pay early.
You should contact them if you wish to pay early. They may accept it.
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.