Firstly, the rules will vary a bit depending on when the time of the forecast is deemed to be made depending on whether you are under NEC3 or 4 and the type of compensation event - as in which compensation event it is listed in clause 60.1.
Having said that, broadly speaking the Contractor needs to make their forecast based on the rules in clause 63 and what is known at the point in time. So if, under NEC4, at the point in time of the 'dividing date', they thought they were going to use Subcontractor A, then that is what they should base their forecast on and you should evaluate it against.
Having said that, broadly speaking the Contractor needs to make their forecast based on the rules in clause 63 and what is known at the point in time. So if, under NEC4, at the point in time of the 'dividing date', they thought they were going to use Subcontractor A, then that is what they should base their forecast on and you should evaluate it against.