State testing in constant flux

We are in the midst of our second year of NECAP testing, the standardized test that Maine adopted along with several other New England states. This follows periodic revisions of the MEA test that was given in the years prior to NECAP. I learned earlier this week that in four years, we would have yet a different test to replace NECAP's. The new test will be aligned to the common core standards which 47 states have (or will) adopt. Apparently, there is going to be a total new feel to the next round of testing, as it will incorporate many best assessment practices. I have no doubt that each new rendition of testing is better than the one before. However, by constantly changing the test, it makes it nearly impossible for a school to use the tests as a measure of improvement efforts over time. We are unable to get any longitudinal test data because the tests aren't comparable. At some point, as a state, we need to determine a constant measurement tool and stick to it! It is tough to garner buy-in when the target is constantly moving.