Employee - Review Rating field: Update to 3-year calculation without closing a review

Content

We would like for the Employee - Review Rating field to show a 3-year calculation.  We would then use this field in a 9-box matrix to show performance over a 3 year period.

We can achieve this by setting Number of Reviews Used in Reference Review Rating to 3 (previously was 1), and then having the latest review set to Closed (or reset to Draft the previous year's review and then reclosing it).

We will not be closing our latest review (2014 review) until February 2015.  We would like to implement the 3-year calculation now, an would like to find ab an easy way of generating the new calculation for the Employee - Review Rating field without going through a process of closing a review (or waiting until February 2015).