I don't have citations handy, but IIRC in general inattentional blindness effects are greatly diminished if you warn people ahead of time, which should not be surprising. I don't know what happens if you warn people between the filling-out-the-questionaire stage and the reading-the-(possibly altered)-answers stage; I expect you'd get a reduced rate of acceptance of changed answers, but you'd also get a not-inconsiderable rate of rejection of unchanged answers.
More generally: we do a lot of stuff without paying attention to what we're doing, but we don't keep track of what we did or didn't pay attention to, and on later recollection we tend to confabulate details into vague memories of unattended-to events. This is a broken system design, and it manifests in a variety of bugs that are unsurprising once we let go of the intuitive but false belief that memory is a process of retrieving recordings into conscious awareness.
It frequently startles me how tenacious that belief is.
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).