Philosophy is a highly discursive discipline founded on argumentative give and take. Often when a philosopher's position is subject to criticism she believes she cannot answer, she modifies her position while trying to retain those elements of those position she believes are most central to it. In other words, the result of receiving criticism is rarely a philosopher 'admitting defeat.' Rather, her position evolves as she strives to absorb the criticisms as much as her extant positions allow.
That said, there are some prominent examples of philosophers who clearly changed their minds over their lifetimes. Perhaps the clearest is Wittgenstein: The 'early Wittgenstein' inspired logical positivism, the 'later' ordinary language Wittgenstein was a critic of positivism. Russell seemed to change his mind a fair bit too. A recent example is John Rawls, who gives a very different foundation for his political liberalism in his early work than in his later work. Kant certainly changed his mind regarding whether human freedom can be demonstrated. And Plato is an interesting case here too: It's tough to know if Plato ever believed the doctrines espoused by Socrates in the early Platonic dialogues, but the views defended in the later dialogues are clearly different.
In short, I suspect most philosophers try to strike a balance between a dogmatic embrace of the views they find plausible and the criticisms of those views, trying to identify the best overall synthesis of these. And in my view, that's as it should be. Philosophy is held back by dogmatism, but it also progresses in part because adherents of particular positions defend them to the utmost.