How strange to meet up through this forum and was it really 4 years ago. Without remembering the details, I am making an intelligent guess that the Performance Bond would have been referenced from Contract Data and not as as part of the Service Information, so the Service Manager would not be able to change it.
Consequently, it can be only be legally changed by a Supplementary Agreement between the Employer and Contractor.
This is not a complicated thing. You just need a piece of paper which references which bit of the contract you have agreed to change and for it to be signed by senior people in each organisation who have the power to enter contracts on behalf of their employers.
Hope this helps,