i think i may have answered my own question - for those of you who are interested:
I cannot find any forms for changing VAT periods on the HMRC website.
However, it does state on HMRC website that if you would like to change your address / tax return periods / other registration details you can do so online if you are registered for online services otherwise you need to write to HMRC.
However you cannot change any details over the phone.
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.