“Step one: rename isn’t enough. TWRP uses a raw dump, but Android’s boot image format expects a header, kernel, ramdisk, and signature.”
If all else fails, you don't actually need to convert—you can create a boot image from your currently running (or previously backed up) system. boot.emmc.win to boot.img
To convert a file (a TWRP backup of the boot partition) to a standard boot.img : “Step one: rename isn’t enough