The Ultimate Collection actually contains more content than the new Legacy collection so changing it would not be an "upgrade" exactly.
The Ultimate Collection contains the IKEA stuff pack and some music/bands that EA no longer have licensing for when releasing The Legacy Collection. If EA had forced the The Ultimate Collection to change to the The Legacy Collection it would be a downgrade and a lot of players would be annoyed.
However an update to The Ultimate Collection (whether that's possible or not, I am not sure) to include any bug fixes that The Legacy Collection has that the former does not would be more appropriate.
