It really depends on the terms attached to the offer. I sometimes find that a new rate ISA comes out on 6 April, so I don't top up ISAs in the next financial year, I take out a new one.
I think you will have to wait until 5th April to see what the new ISA rates are for that tax year. Might be worth waiting and getting a better deal then for one, two or three years cash ISA. Google 'Best ISAs' for the best deal and after the year/two years is up make a note to contact them and change your ISA to the best deal again otherwise the provider will automatically put your money into their lowest account. They should remind you by post when the deal is finishing and you can decide what to do then.
I have just opened (few days ago) one of these 2 year fixed rate with Santander at 2.8% and put the maximum in for THIS tax year (2012/13).
The paperwork that came with it says if I put in the maximum for NEXT tax year (2013/14) by 30th May I think it was, you get the 2.8% on that money for the full 2 years as well.