why did they change wrath to pride in the 2003 anime.

When the 2003 Anime (Anime1) was in production, the manga had not reached the middle point of Act 1 (I think the chapter at the time was the Brothers Elric's reunion with Izumi). Adaptations of mangas either stopped at a certain point or continued on, sometimes making loads of filler arcs to give the manga time to make more plot to adapt. FMA author Arakawa had given the animators her blessing to make a conclusion to the series, with an ending different from her plans for the series.

I believe at this time, Bradley had not yet been referred to as Wrath, and the present chapters of the manga probably gave no hint of his anger issues. And the true identity of Pride would not be revealed until a few years later (c. 2006). Thus the Anime1 creators called Bradley Pride (makes sense, since he is the absolute ruler of the country. DEFINITE Pride there) and created a new person called Wrath.

Pride-Bradley seemed more in control of his country than Wrath-Bradley, who was more a pawn for his master and knew it. So the name-change makes sense given their change in status from each anime.

When darkness overcomes the heart, Lil' Slugger appears...