>For Windows, you'll definitely need a reinstall. There is this base assumption that UEFI=GPT and that BIOS=MBR which is only partially true (at least for wind

>For Windows, you'll definitely need a reinstall. There is this base assumption that UEFI=GPT and that BIOS=MBR which is only partially true (at least for windows 7/8/8.1/10). In fact the UEFI is perfectly capable of booting from both GPT and MBR disks, the issue is only that when you install an OS in CSM (or BIOS) on a MBR disk, the OS loader/boot manager installed will be the one needed for BIOS booting, and the UEFI one won't be installed. Besides there is the requisite of having a FAT32 (this can usually be worked around with a UEFI NTFS driver). But a "normal" BIOS install on MBR (with a dedicated, without drive letter automatically assigned, FAT32 primary partition dedicated to the boot files) can be booted just fine in UEFI by simply adding to that partition the needed UEFI loaders. The opposite (booting with BIOS from a GPT disk) is as well possible but needs some non-standard modifications/hacks to the disk MBR, that - although proved in the years to be "safe" remain a sort of hack (not recommended if not really-really needed).