About AS and A2Watch
If you swap exam centres between AS and A2 you need to make sure that the second school enters you for the exams with the UCI (unique candidate identifier - 13 character code, e.g. 12345b152222A) the AS centre used or Edexcel won't see the results as coming from a single candidate.
There's no such thing as an A2 certificate - it's just the second half of the A level. To get an overall grade (at AS or A level) you need to ask the centre to cash-in the qualification. All this means is that they send a code to the exam board (normally at the same time as they enter you for the exams but it can be done until a month or so after the results come out) which tells the exam board they need to add up your best attempt at each unit and allocate a grade based on that. If you want grades for both AS and A level you just cash-in both. The exam board doesn't charge for this if it's done at the time of entering for the exams, there's a modest fee (around £5) if it's done after this.