A Level Credit TransferCIE says:
The AS Level and 'A2' syllabuses of the different English awarding organisations tend to be very similar in content because they are written to comply with the requirements of the English regulator, Ofqual, including 'common core' requirements as to content and the number of units constituting an AS Level and an A Level. English awarding organisations are therefore usually able to accept one another's AS Levels to join to their own A2 units to constitute an A Level.
Cambridge's International AS Level and A Level, in contrast, are designed for international use, are not unitised, and do not need to comply with the requirements of Ofqual as to content, since they are not intended for use in schools in England that are state-funded. However, it follows that English awarding organisations are not allowed to credit an AS Level from Cambridge in lieu of one of their own AS Levels in determining whether a candidate has done the correct units for an A Level award.
It also follows that the reverse, adding a Cambridge 'A2' to an AS Level from an English awarding organisation, would be likely to produce an incoherent overall course, and Cambridge always advises that students who transfer from a school or college where they have done AS Level with another organisation should take the whole of Cambridge's A Level rather than seeking to add on just the 'A2' part. One reason for this is that Cambridge International A Level syllabuses are not strictly modular and the ‘A2’ examinations will presume a knowledge and understanding of the Cambridge International AS Level syllabus.
We do, however, consider exceptions but they entail a judgement by one of our senior Examiners in the subject about whether the amalgamation of the other board's AS Level and the second half of our own A Level course produces a coherent whole. We charge a fee per subject for this purpose, and clearly cannot guarantee that the outcome will be that the combination is acceptable. We will attempt to complete consideration of any such applications within a month, but this may not always be possible if the request comes during an examination marking period.
In order to proceed, we would need to know which units the student took, with which awarding organisation, and in which examination series. If we felt able to allow the substitution, we would obviously also need to know the uniform mark awarded for each of the units constituting the AS Level. We would have to check this information with the Awarding Body concerned. We cannot credit individual AS Level units towards an A Level, only a complete AS Level.
Please note that all applications must be sent to us by the candidate's school. We are unable to accept applications from candidates or parents directly.
Edexcel says transfer both ways is possible, done through CIE.
OCR says only possible for AS maths, from any exam board.