United Continental Holdings Inc. is overhauling an incentive program to give employees a chance to win $100,000, a luxury car or a swanky vacation. One union leader says his membership would rather have a shot at bonuses that are far, far smaller — but also a much surer thing. Under the new approach, one lucky winner will take home the six-figure cash prize every quarter, with as many as 10 people getting Mercedes-Benz C-Class sedans or $40,000 each, President Scott Kirby said in a memo to employees Friday.