According to the
Lloyds TSB Fixed-rate ISA webpage, the 6.5% for balances of £9k+ is good until May 31st. In theory, that would mean you could pop along to Lloyds TSB on April 6th, and get them to transfer your two years' worth of NS&I ISA in addition to next year's ISA allowance (which I understand will be £3600) to make a balance in excess of £9k which will then qualify for the 6.5%.
Alternatively, you could add, say, £3600 to your NS&I ISA, then go into Lloyds at some point prior to May 31st and ask them to transfer the whole thing (or at least £9k of it) into a Lloyds TSB FRISA (Fixed-rate ISA).
As I understand it, when transferring an ISA, you specify the %age of the ISA you'd like to be transferred (which could be 100%, in which case, I assume, the other party -- NS&I in this case -- would close your ISA with them).
I hope this addresses your questions.