Thank you for your reply Matthew (?), I managed to get through to HMRC helpline at last and they say:
As an example, in my case my current VAT accounting period runs from Feb to end of April. The new FRVS criteria (limited costs trader rules) starts beginning of April. I should write to tell them of my intention to move out of the FRVS, if accepted they write back giving me authority to use FRVS (in my case at 11%) until end of April. That’s it, a shame but quite straight forward in practice.