Our client wishes to commence a defined part of the works under an Option E contract in order to inform and then determine a fixed price for the remainder of the works. If the initial works are commenced under Option E can the price then be switched to a fixed lump sum?
In practice "Yes’ BUT it would have to be by a Supplemental Agreement to change it, all details would have to be agreed, written down and signed by both Parties. That would include the Prices which would become a meaningful figure for both Parties.
I would suggest that you agree as much of the details as you can now including the criteria for when/what state you make the switch and how the Prices etc are agreed.