With ECS option A you get paid the lump sum in the activity schedule for each activity once that activity is completed, see clauses 50.2 and 11.2(27). If the activity is not complete you do not get paid anything at all for it. Therefore, if the activity is complete, as defined by the final sentence of clause 11.2(27), you get paid the full amount set out in the activity schedule.
The prices in the activity schedule can only be changed in accordance with the contract, for example in the case of compensation events, see clauses 60 to 65. Even then, the processes in the contract need to take place before those changes can take place.
You should speak to the contractor and ask why the revaluation has been done. If you cannot reach agreement, your ultimate remedy is to take the dispute to adjudication in accordance with the provisions of option W2.