Technically it wouldn't, because Sora locked the portals that connected to the past. Therefore, if he eliminated all the Heartless in that world, it's safe.
Here's the quote from the interview, in case you wondering:
[QUOTEInterviewer: Are there new enemies you can confirm that are not shown in the screens?
Nomura: The new enemy is not a Heartless, or a Nobody, because the story takes place before even they appear. I can only give out more information, little by little, as time passes: an example being like the Shadow enemies that are heartless, and the Dusk enemies that are nobodies.
][/QUOTE]