Global maintenance/ court order

Clearchem

As global maintenance has to include a substantial sum of spousal maintenance I seems fair to assume from your information that there is £250 per child and £500 for you. The change is not legal, the reduction too much and you could return to court for enforcement. Expensive. There is a risk they reduce the spousal element.

You could also apply to the cms. If he earns 70k, assuming a reduction of pension @ 9% the liability for 1 child is £637 per month if no other adjustments. Global sum order reduce to take into account future cms calculations so my logic is thus:  payable would be £1000-250(for the child that has moved out)-£250(as a cms assessment is now in place for the remain child)+637(the cms assessment)=£1137 monthly once the older child turns 18.  You would still loose your tax credit for 1 child, and you should stop this before it is fraud.

Noting that he could also apply for cms against you for the next 6 months. You liability appears to be £130 per month for this period

You could suggest a family arrangement of continuing the £1000 or you will take steps to enact the above, which after 6 months will be more costly.