In an NEC Option A Contract the activity schedule is simply used as a way to calculate how much the Contractor is to be paid each month. i.e The amount due and the Price for Work Done to Date is determined by completed activities on the activity schedule.(See main option clause 50.2 and Option A clause 11.2(27).
The activity schedule can’t be used to deal with re-measure or provisional sums. A change in the activity schedule is not one of the compensation events listed in clauses 60.1(1) to 60.1(19).
If this is something you want to do the best way is define the quantity of rebar for remeasure is to insert the quantity into the Works Information. Then when the actual quantity is known the Project Manager can change the quantity by giving an instruction to change the Works Information under clause 14.3. This will result in a compensation event under clause 60.1(1).
The way the change to Prices are assessed are detailed in clause 63.1. I don’t believe there is any allowance for loss of profit under that clause.
I hope this answers your question, but if not, please don’t hesitate to get back to me.