We hold regular QAT (Quality Action Team) meetings with customers and OEM's for which we act as an authorized repair vendor.
One of these OEM's requires that all test equipment used for final acceptance of any of their products be calibrated at six month intervals. (The normal cycles are one year to eighteen months)
While researching the source of this requirement recently, I found that the only documented evidence was an Action Item in an old QAT Register.
My question is this. Is an Action Item resulting from a meeting proper authorization for making a significant change in standard policy ? Shouldn't the OEM have been required to either submit an addendum to the contract or at least provide some formal writing ?
OR, is our QAT meeting a valid medium through which system changes can be made based on a customer request ?

You've lost me. You say a customer requires a 6 month calibration cycle. You state a 'normal' cycle is 1 year to 18 months. I'm not sure what you mean by 'normal'. Calibration cycles are determined by use, etc. Any time you require a more frequent calibration than use, etc. requires, it's surely not a 'problem'. As far as whether such a meeting is 'proper authorization' - that depends upon your company calibration system. Annotate your calibration database record to indicate the reason for the shortened cycle. Also consult NCSL's RP-1 re calibration cycles.

The meeting should be satisfactory 'authorization' to shorten a cycle. Lengthening the cycle would be a problem.