what is the aacei or other official definition of "material change"? as in important not a material like wood...

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12 years 6 months

Here is an answer - but term is difference in AACEi's glossary

"Material Difference" - A change that is important to the performance of the work or that will have a measurable influence or effect on the time, cost of, or procedures for the work under the contract.  

It is interesting that is said nothing about critical path or changes to the substantial completion date.