hint: this one does exist in the GG FLAT TIRE
the (very) short answer is as follows:
FLAT TIRE RULE:
CUSTOMERS CONTACTING UNITED WITHIN 30 MINUTES PRIOR TO DEPARTURE OR 2 HOURS AFTER DEPARTURE MAY BE REBOOKED AT NO CHARGE ON UA/UAX FLIGHTS. ALL SEGMENTS OPERATED BY OA SHOULD ONLY BE REBOOKED BY THE OPERATING CARRIER AS THEY MAY BE SUBJECT TO ADDITIONAL FEES