In principle, though, I don't like the practice and it should be undertaken in only special circumstances. People ought to know what they're getting when they pull the lever. I sure as hell wouldn't want an R suddenly going D on me. What if I'd contributed?
That is true; however, under certain circumstances, it's not the wrong thing to do. It sounds like Mr. Griffith is (a) more conservative than the average Democrat, and (b) very tuned into what's going on in his district, and realizing that it turns out that his constituency is growing more and more conservative as time goes on; under those circumstances, it may very well make good sense to get out ahead of the curve and get to where his constituency is going first so he can be there to greet them.
As far as it going the other way, I've said ever since Specter pulled his little trick that it was better that he jumped ship voluntarily than that we had to spend a lot of time, money, and scarce political capital forcing him out of his seat. Now, instead of having to worry about ruffling the "senior senator's" feathers, we can pick a good candidate in a competitive primary free of Specter-taint, and then well and truly kick his ass so hard his intestines pop out of his mouth.