The answer is not going to be as simple as you think! It will depend on the status of the information about the piles given at tender stage. If this was incorporated as part of your Works Information for your design (as stated in Contract Data Part 2) then it is part of the Works Information that you are require to work to, and only the PM can change that. So you would not be able use the 500mm piles, and therefore the change to the loading would have no effect upon the Actual Cost or time for carrying out the work. If your design for the 600mm piles were inadequate (and you needed 900mm piles) then the PM could issue an instruction at your request changing your WI, but that would not be a compensation event because of the exceptions in clause 60.1(1).
However if this design information was not part of the Works Information then the answer is different. You are entitled to design the works in whatever way you want and you would be able to use whatever sized piles were suitable as long as they complied with the WI, i.e. met the loading requirements, whether they were the same, larger or smaller than those you allowed for in your price. If you changed them because your initial assessment of the size was wrong (whether too big or small) that would not be a compensation event, and it would not change the price. The change to the loading of the piles instructed by the PM is a change to the WI which is a compensation event (60.1(1)). That CE is assessed based upon what effect it is going to have on the Actual Cost for carrying out the works (63.1). In making that assessment you ignore whatever price is in the activity schedule and base it upon what is was going to cost (in your case with the 500mm piles) and what it will now cost (with the 600mm piles). But remember that is only IF the original information on the pile size was NOT part of your Works Information.’’