These two are very similar qualifications and pretty incestuous.
Give the AAT a call on 0845 863 0802. They're pretty friendly people on the help desks.
Alternatively, for AAT you need to have a training provider and generally they will give you advice as to whether you would be allowed to swap over.
Try Premier Training (info@premiertraining.co.uk) as they're quite friendly and very reasonable.
You would also need to sign up with the AAT rather than CAT so you're going to have another expense there (£107. £72 subscription plus £35 admission fee).
Hope that this helps as a starting point for you.
cheers,
Shaun.
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Shaun
Responses are not meant as a substitute for professional advice. Answers are intended as outline only the advice of a qualified professional with access to all relevant information should be sought before acting on any response given.