How to Change Legacy to UEFI Without Reinstalling Windows 10: A Guide

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Changing an existing Windows 10 installation from Legacy BIOS mode to UEFI without reinstalling can be possible when the system disk and hardware meet the requirements. The key is to convert the system disk from MBR to GPT correctly before switching firmware boot mode. Back up the PC and save the BitLocker recovery key first.

Validate before converting

  1. Open System Information with msinfo32 and confirm the current BIOS Mode.
  2. Open Disk Management and identify the actual Windows system disk.
  3. Make a current backup and suspend or otherwise prepare BitLocker according to Microsoft’s guidance if encryption is enabled.
  4. Use Microsoft’s mbr2gpt tool to validate the system disk before making changes.
  5. Proceed with conversion only if validation succeeds and the hardware supports UEFI boot.
  6. After conversion, restart into firmware setup and change the boot configuration from Legacy/CSM to UEFI as required by the manufacturer.
  7. Start Windows and verify UEFI mode, boot operation, encryption status, and recovery partitions.

Do not switch firmware mode before the disk is ready

An MBR Windows installation configured for legacy boot can stop starting if you simply disable CSM and select UEFI. Likewise, disk conversion without the corresponding firmware change can leave the PC unable to find Windows.

Conversion has prerequisites

MBR2GPT checks disk layout, partition count, available space, and other conditions. If validation fails, investigate the reported condition rather than forcing partition changes. Complex dual-boot or vendor recovery layouts deserve extra caution.

Why make the change now?

UEFI and GPT may be prerequisites for Secure Boot and Windows 11 migration. However, Windows 10 standard support ended October 14, 2025, so if the PC meets Windows 11 requirements, plan the firmware conversion as part of moving to the supported OS rather than as an end in itself.

After a successful conversion, check that Windows Recovery Environment still works and that the correct Windows Boot Manager entry is first in the UEFI boot order. Keep the backup until several normal restarts have succeeded.

Prepare the disk before changing firmware boot mode

A Windows installation using an MBR system disk normally needs a valid GPT conversion before the firmware can boot it in UEFI mode. Microsoft’s MBR2GPT tool can validate and convert eligible system disks, but it has prerequisites and is not a substitute for a backup. Validate first and stop if the disk layout is unsupported.

Keep recovery information available

Save the BitLocker recovery key, create a verified backup, and record the original firmware settings before conversion. Do not switch the firmware from Legacy/CSM to UEFI until the disk is ready. Secure Boot is a separate setting and does not need to be enabled at the same moment as the boot-mode conversion.

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