R’ Moshe Feinstein said that one can switch from davening sfard to daven ashkenaz, but not the other way around.
When I was in college, I learned in a small yeshiva for a year, where one of the Rabbeim was a student of R’ Aharon Kotler. He told us a story where R’ Aharon was traveling with a few students (including this Rebbe) and they stopped to daven Mincha. The shul davened sfard. R’ Aharon asked if he could daven for the amud, and he went up and davened ashkenaz. One of hte students asked him, “But Rebbe, the minhag hamakom is to daven sfard!” He replied, “Minhag hamakom has its place, but this is the proper nusach!”
Ah, good times in that yeshiva. It was where I met Feif Un. We went to the same college and the same yeshiva while in that college!