The Fulton Street station-plex is an amalgamation of four different subway stations, built at different times – the East Side IRT (4 & 5), the West Side IRT (2 & 3), the BMT Nassau Street Line (J, M, Z), and the IND (A & C). The nomenclature for all of them is “Fulton Street”, except for the IND, which is “Broadway-Nassau”.
That makes sense, as all the lines except the IND, run perpendicular to Fulton Street, and thus intersect it. The A/C, runs along Fulton, and the streets it intersects with, are Broadway on the western end of the station, and Nassau in the easterly direction.
That logic holds on the Lower East Side, where the J/M/Z stop is Essex Street, and the IND station below it, Delancey; likewise in Queens, the IRT stop is called 74th Street, and the IND, Roosevelt Avenue.
Apparently as part of the work, some renaming is taking place on the IND platform.
On the eastbound wall, many of the “BWAY NASSAU” tiles have been replaced with tiles that say, “FULTON ST”. The westbound tunnel wall, is so far, unchanged. For some reason, not even all the eastbound tiles have been replaced. Whether this is due to some plan, or just to keep tourists on their toes, I don’t know.
So for now, when your train pulls into the station, its name depends on which window you happen to look out.