How do you account for him changing his mind when the argument has been the same for 50 uears.
Oh, gee, I don't know ... maybe he finally had enough evidence/experience to convince him that he just couldn't justify those old arguments anymore, and was honest enough to say so.
I can say with some authority that it happens, as that's what happened to me ... though admittedly I only had about 20 years of those sorts of arguments to undo.
I would go further, and venture to guess that Mr. Flew had been having his doubts for quite a while before he "came out," and that his pride made it difficult to admit his conversion.
But of course, it's much easier to dismiss Mr. Flew as senile. And, of course, ad hominem claims of senility are also much less dangerous for those who might begin pondering their own doubts about their atheist beliefs.
(I suppose it's mere coincidence that the Soviets were similarly prone to questioning the mental capacity of those who dared to question Soviet orthodoxy....)