Had they known what was coming next,they - second thoughts

因为must have +过去分词,是对过去情况的推测,那是要根据事实来推测的.
而此题是考查虚拟语气.
Had they known ...是对过去情况的虚拟,相当于If they had known ...
根据过去情况虚拟语气的构成：
if从句 主句
had+过去分词 would/should/could/might+have+过去分词(根本没有用must的情况）
可知此空要填：might have had 而不能填：must have had .
句意：要是他们早知道接下来所发生的事,他们可能已经改变想法了.
（实际情况是：他们不知道接下来所发生的事,因此他们并没有改变想法而重新考虑.）