DaMoshe,
Interesting that it’s said that the Bal Shem Tov actually davened Nusach Ashkanaz, based on the siddur he used.
The topic of Nussach is complex, one cannot just change things around and there are lengthy discussions about this in the leading poskim.
Some say that the current Nusach Sefard dates back to the times when every shevat had their own nusach, which they had in their mesorah that targets another shaar. There was one nussach that was dedicated for those that are unsure as to what shevat they belong to, this is the nussach that is nowadays called nusach sefard.