A note of introduction: B'Mareh HaBazak is a collection of responsa written by the scholars in Kollel Eretz Chemdah, an institution in Israel dedicated to producing high-quality Talmidei Chachamim and judges. The teshuvot are unsigned by any particular scholar.
In a teshuva wrutten in 2005, the community in Pforzheim, Germany asked whether they could change the nusach that is used for davening in their shul. As the shul was originally populated by Jews of Middle Eastern descent, the nusach used was that of Eidot HaMizrach. However, over the years the community changed in its make-up and a new building was built, and the question posed was whether they could switch to using nusach Sefarad, as per most communities in the Land of Israel.
The initial position taken in the teshuva is that the nusach of a shul should not be changed, certainly so long as original members of the shul remain. If people with a different nusach join the shul, they should daven to themselves with their own nusach, but the chazzan should use the official nusach of the shul.
However, in this case there was a desire to change the nusach not only to accomodate the newer members of the shul, but also as a way to draw more people into the shul who might otherwise not be affiliated with any shul (i.e. kiruv). As such, given those realities, plus the reality of a new building, there seems to be room to allow the change. It is recommended to also change the name of the shul, and to perhaps introduce some new by-laws, all in the name of creating a sense that this shul is a new entity and thus the change is part of a larger shift in the overall composition of the congregation.