What happens if an index is changed after it has been used in calculating a Price Adjustment Factor?

NEC3 and NEC4 Contracts Secondary X, W and Y options
1

What happens if an index is changed after it has been used in calculating a Price Adjustment Factor?

2

If an index is changed after it has been used in calculating a Price Adjustment Factor, the calculation is repeated and a correction is included in the next assessment of the amount due. The Price Adjustment Factor calculated at the Completion Date for the whole of the works is used for calculating price adjustment after this date (see Clause X1.2).