Agree with the above sentiments that the vendor should be stepping in with helping out given the mutual benefit for all concerned..

That said chances are the affiliate ID is a get parameter on the URL e.g. domainname.com?affid=xxx ... If that is the case then you can change the URL with making sure to add the get parameter to the end of it which is the part including the question mark and after it e.g. domainname.com/landingpage?affid=xxx