There are differences in both the hardware and the firmware between the Geo2/2.1 and the Geo3 boards, some of these changes are obvious, some less so. It is possible to put a Geo3 board into a Geo 2.1 however it will not control the Geo2.1 in the optimal way, which could lead to velocity and efficiency issues and these issues might not be immediately apparent.
Steve 'Flash' Monks
Planet Eclipse HQ
In theory, there is no difference between theory and practice. In practice, however, there is.