Yes, you have answered your own question. Cl 63.1 confirms the date the assessment should be based on and clarifies the switch point from using actual Defined Cost to forecast Defined Cost.
Your question is interesting, how did the PM provide new Works Information if it was not under instruction. Contractually the Contractor is to provide the works in accordance with the Works Information so if the Works Information has not been changed under the contract then it should continue to work to the original. This emphasis the importance of timely notifications by all parties.
If it is accepted the Contractor was working to the new WI prior to the formal instruction then the CE (change in WI) should be based on actual Defined Cost to the date of the PMI and forecast Defined cost from that date.
Your question is interesting, how did the PM provide new Works Information if it was not under instruction. Contractually the Contractor is to provide the works in accordance with the Works Information so if the Works Information has not been changed under the contract then it should continue to work to the original. This emphasis the importance of timely notifications by all parties.
If it is accepted the Contractor was working to the new WI prior to the formal instruction then the CE (change in WI) should be based on actual Defined Cost to the date of the PMI and forecast Defined cost from that date.