If a candidate has done A-Levels before, but now decides to change to AP's or even another exam, does that candidate have to declare all his other (GCSE, AS, etc..) exams grades too, even though they've changed systems? Bear in mind they have been certified.
Why is that though? I mean the candidate has changed systems and therefore the old examinations should not count.
Because you are asked to declare all qualifications (and sign, or 'tick', a declaration to that effect stating that you have answered truthfully and fully), you aren't just asked to declare pertinent qualifications.
AS levels are qualifications in their own right, and obviously so are GCSEs, so if they're certified, they have to be declared. Unis won't just want to know how you're doing at 18, which could now be APs, but also how you did at 17 (AS levels) and 16 (GCSEs) to give them a more complete picture of your academic record, rather than just a snapshot.