How will this affect the use of the CPI to index contracts and rates? Is the current CPI still fit for that purpose?

Yes, the current CPI is still the benchmark for the official inflation figures that can be used for contracts, and that benchmark status will continue after the new method is introduced. The transition to the new method will be scheduled in such a way as to ensure that CPI-based indexation will achieve an average multi-year outcome at a level that would have applied if the new method had been introduced earlier.