They wouldn't necessarily need to change who they are by adding someone who could be a situational rotation player. The playoffs are a different game than the regular season, it only makes sense to have as many tools in your toolbox as possible when playing a whole series against a specific team.
Also, I think they are good enough to make a serious run now, and in general I don't think teams should wait to make their teams better, especially when they have as many assets as OKC does. Rebounding is their glaring weakness, and I think it will become a problem in the post season where extra possessions are even more valuable. They are currently 28th in the league in rebounding %, so they really have a long way to go.