Unlike the original Final Fantasy XIII, Final Fantasy XIII-2 will be getting quite a lot of DLC. So much that the DLC packs will be released on a schedule following the game's release.
Yoshinori Kitase, FFXIII-2 producer, said that they plan to release 1 to 2 DLC packs each month containing multiple items. These items could be weapons, costumes, or enemies and Kitase said that this content will have to be obtained, not just downloaded. The content will be mixed in with gameplay meaning you may have to defeat a special boss in order to have access to your downloaded content. Kitase has also confirmed that all of these items will show up in event scenes unlike prior Final Fantasy's where only the character's original items would show up.
Hopefully we'll get some sneak peeks at these costumes in the near future as Final Fantasy XIII-2's release date is coming up fast. Perhaps we'll see Serah dressed up in Aya's clothing? Who knows?