There are no specific "cut off" points - it depends on earnings and on things such as the number of children, actual childcare costs, hours worked etc.
If the pay increase has only just happened it will not have much effect on the total income for the 2008-09 tax year, so it should not affect the current payments very much.
BUT you must tell them now about the change and what the 09-10 income is estimated to be. If you don't, they will go on for some time paying you on the basis of the old income, and when they eventually get the new figure you will have been overpaid and will have to repay it.
After you have told them of the increase they should issue a new award notice including the amounts they will pay for 09-10. Check it carefully to make sure all the figures are correct. If they don't send one, keep chasing them until you get one.