Unlike major feature patches, Patch #3 (v4.1.1.3732833) is safe for ongoing saves. The Magic Mirror alone is worth the 2.5GB download. While it doesn't add new story content, it sands down the rough edges of the final act and gives you the cosmetic freedom the community begged for since Day 1.
The update also introduces several critical balance changes to streamline the D&D-inspired mechanics: Baldur's Gate 3 Patch 3 Notes - Baldur's Gate III Guide