Convert to BMP only if you need compatibility with very old software, specific professional applications, or legacy systems that can't handle modern formats. For most users, converting HEIC to JPG or PNG is more practical due to much smaller file sizes.
BMP files will be 10-20 times larger than HEIC because BMP is completely uncompressed while HEIC uses advanced compression. A 2MB HEIC photo might become a 30MB BMP. This makes BMP impractical for most users.
BMP will preserve the quality that exists in the HEIC file, but it won't recover any quality lost during HEIC's compression. Since HEIC uses lossy compression, some original detail is already gone. BMP just stores this without further compression.
Use BMP only for specific legacy workflows, certain professional applications that require it, or when working with very old software. For general photography, storage, and sharing, JPG or PNG are far more practical choices.
BMP is a very basic format that only supports standard RGB images. It doesn't support HEIC's advanced features like transparency, HDR, or depth information. You'll lose these features during conversion.