I know a guy who "has always been religious" ever since he almost killed himself in a car crash.
My wife, not long before she met me, became an instant non-smoker and was genuinely surprised when friends offered her cigarettes -- she had to make a conscious effort to recall that she had previously smoked, because it was no longer consistent with her identity, as of the moment she decided to be a non-smoker.
This seems to be such a consistent feature of brains under self-modification that the very best way to know whether you've really changed your mind about something is to see how hard it is to think the way you did before, or how difficult it is to believe that you ever could have thought differently.
It's the best way I've seen to quit smoking - it seems to work every time. The ex-smoker says "I'm a non-smoker now" and starts badmouthing smokers - shortly they can't imagine doing something so disgusting and inconsiderate as smoking.
Change blindness is the phenomenon whereby people fail to notice changes in scenery and whatnot if they're not directed to pay attention to it. There are countless videos online demonstrating this effect (one of my favorites here, by Richard Wiseman).
One of the most audacious and famous experiments is known informally as "the door study": an experimenter asks a passerby for directions, but is interrupted by a pair of construction workers carrying an unhinged door, concealing another person whom replaces the experimenter as the door passes. Incredibly, the person giving directions rarely notices they are now talking to a completely different person. This effect was reproduced by Derren Brown on British TV (here's an amateur re-enactment).
Subsequently a pair of Swedish researchers familiar with some sleight-of-hand magic conceived a new twist on this line of research, arguably even more audacious: have participants make a choice and quietly swap that choice with something else. People not only fail to notice the change, but confabulate reasons why they had preferred the counterfeit choice (video here). They called their new paradigm "Choice Blindness".
Just recently the same Swedish researchers published a new study that is even more shocking. Rather than demonstrating choice blindness by having participants choose between two photographs, they demonstrated the same effect with moral propositions. Participants completed a survey asking them to agree or disagree with statements such as "large scale governmental surveillance of e-mail and Internet traffic ought to be forbidden as a means to combat international crime and terrorism". When they reviewed their copy of the survey their responses had been covertly changed, but 69% failed to notice at least one of two changes, and when asked to explain their answers 53% argued in favor of what they falsely believed was their original choice, when they had previously indicated the opposite moral position (study here, video here).