Interesting that, despite around 3 pages of posting, we have actually heard from precious few of those people who actually believe.
According to the Pew Research Institute in the USA - a pro-faith think tank- switching religions is very common in the USA - I have seen them claim as high as 50% of all believers! which I find difficult to believe. Other studies I have seen suggest it is far less common, but there is little actual evidence out there.
Keyplus claims to have studied alternative religions -"including atheism",no less- in his mid-20s, but despite this exposure to different belief systems has elected to stick with the one he was brought up in.I think reviewing your faith like that is uncommon amongst believers, especially fervent ones, but that among those that do, they will change, usually prompted by a real world event.
It would still appear that the initial thesis behind naomis question holds though; Those with a self-professed faith generally were brought up in that same faith. So if you were brought up a christian you will be a christian in adult life, brought up a muslim you will remain a muslim in adult life, and that points to a couple of things - that stuff we learn as a child tends to shape us as individuals and influence us throughout adult life, and second that it is difficult to break such early influences and conditioning.