Also to further clarify Bill's comment, and my comment "removing mistakes of the past", there are many instances that involve that.
For example, some of these "mistakes" were actually in original design, such as at Fulton, Broad, and 8th Ave that were designed to look like IND stations, but now have been "corrected" with "reproduction" mosaics to match the rest of the stations on the lines to finally look like "BMT stations":
8th Ave got the "Canarsie Line" mosaic that is seen throughout the rest of the line:
Fulton and Broad got a Grecian design that is seen at other Nassau Line stations:
Other instances include things like the Canal St/Lex Line reproductions that imitated the 1930's tile over the horrible, totally clashing 1950's tile on the extension, as seen in my example in the original post in this thread.
Then there are places where they really corrected the "mistakes of the past" such as removing the foolish 1960's tile on the Broadway Line in Manhattan that covered over that lines intricate mosaics.
Finally, there are many examples throughout the system where they have painstakingly recreated mosaic bands and name tablets that were lost to either construction, water damage, etc.