One of the biggest misconceptions in the anime community is that Fullmetal Alchemist has a remake. It actually has two separate adaptations.
Fullmetal Alchemist (2003) was made while the manga was still ongoing, so it eventually diverges into an original story. It starts similar, but later becomes its own narrative with a different tone, villains, and ending.
Fullmetal Alchemist: Brotherhood (2009) is a faithful adaptation of Hiromu Arakawa’s manga. It moves faster at the beginning since some early arcs were already adapted in 2003, but then fully follows the manga’s storyline, worldbuilding, and ending.
2003 = original story, more introspective, anime-original ending
Brotherhood = faithful adaptation, faster start, full manga story & canon ending
Both are amazing, just very different experiences. Which one did you watch first?