When you export and import an entire roster, you will lose anything from the previous roster. For example if you were working on the 1988 roster and imported the entire 1990 roster, all the work on the 1988 roster would be erased and you would now be working from the 1990 roster. If you want individual players imported into the 1988 roster from the 1990 roster, you would have to export the players individually from the 1990 roster then import them into the 1988 roster.
When you go to export all players, you should have to name the CSV file that it creates. After that you can open the editor with a different roster with the same number of players as the roster that you exported and import all players and have the exported roster appear. For instance I exported all the players from the M16 roster and named the file. I then opened the editor with a M15 roster that had the same number of players and imported the M16 roster so that I had the M16 roster for Madden 15.
Unless you are trying to import an entire roster to a different game, you are best off just importing players individually from another roster. Also if you import more than around 500 players from another roster, fantasy draft will no longer work completely unless you change the player Id's to reflect the players you deleted from the default M25 roster. If you use the replace player option instead of just importing, fantasy draft will continue to work. If you are looking for more details about fantasy draft and player Id's, click my username and search my posts for a more detailed answer. I'm using my phone right now so it is difficult to copy and paste.

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