Although I encourage you to pursue your strengths and passions, even if it means changing fields, I also advise some caution if you do not yet have tenure. If you feel you can still do more work in your current field that will help build a strong tenure case, you should make sure you continue to devote the bulk of your efforts on that. And I would also be wary of telling people about your plans to change fields (of course, it's a little too late for that advice). Stuff like that can really damage an otherwise strong letter of recommendation: "So-and-so has done brilliant work in .... but has told me that he intends to shift his research away from that and to .... Unfortunately, I cannot comment on the latter."
But if you feel certain you have reached a dead end in your current direction of research, for whatever reason, then you should of course shift full force into something new. Even then, I encourage you to make the initial shift into a direction that can be somehow sold as a natural outgrowth of your current research. Later, when you're a tenured full professor, you can do whatever the heck you want.
