You can convert your partitions from MBR to GPT without reinstalling Windows completely: no need to reformat your drives.
If Windows does not let you do that for some reasons in its very basic "Disk Manager" (limitations in the possible configurations recognized, or stupid restrictions in its UI that does not support any "advanced mode", not even with some warnings), you can do that with free tools like "Minitool Partition Wizard" (which is a lot simpler to use that the builting Disk Management in Windows, and that even blocks you for performing some changes that are still supported by the kernel and filesystem drivers, or that does not offer options only available with complex options using command line tools like Diskpart or Powershell commands).
Note that the free version of "Minitool Partition Wizard" is usable for almost all options, except a few ones mostly used on servers (like dynamic disks, or changing the cluster size, or changing some attributes, or advanced options for RAID arrays and SAN disks), or for some recovery options, for which you need to buy the "Pro" version (though it's not expensive); but it is not intrusive, and won't constantly popup you with ads; and unlike other commercial third party disk management tools, it also has no time limitations, or limitations on volume sizes or number of partitions.
You can also do these changes using some bootable Linux Live CD or USB (note that when you change the boot order, or if you enable CSM, the current TPM automatic boot will be disabled, and the next time you boot Windows in secure mode, it will enter into TPM recovery mode requiring the existing TPM or password/pin recovery or external USB: this is normal behavior which also occurs if ever you have unplugged and replugged in the TPM module. (Microsoft explain the conditions under which such revalidation will occur: don't loose your TPM recovery backups! If you are not connected to a Windows domain, Microsoft also proposes to recover your Bitlocker encryption keys from your online Microsoft account using 2-factor authentication, but this requires you to register your administrator account on the PC with a Microsoft live account and not jsut using local account; in enterprise domains, you can recover your Bitlocker encryption keys also by using Active Directory but you need to contact your domain admin to reauthorize your PC and allow it to boot again in secure mode).
One difficulty wit hthe new stricter requirements is that they not only want you to boot in secure mode, and with UEFI, but CSM mode must be disabled in BIOS: this has the effect of disabling many devices in BIOS, including some external storages (including some hardware RAID adapters), or display adapters, whose option ROM offers no EFI driver for secure boot: these devices must not be needed to boot your systeml but can be used with Windows drivers after booting Windows successfully: you need recent drivers for these devices that don't need any BIOS EFI support. This can be a problem with some devices that use third party dus drivers: make sure you also update the bus drivers for your motherboard and that you have flashed the latest BIOS versions with proper EFI boot support that do not enable any "legacy devices".
This includes notably SATA controlers, USB controlers, and sometimes you'll need to select the correct display driver as your primary display for boot (check which display board is usable: the motherboard's discrete GPU or the third party display board and note that many legacy third party display boards don't even have EFI support in their BIOS and will only display at boot time in legacy SVGA mode, without acceleration support!). You may also need to disable audio devices in BIOS (they will only be enabled after boot using drivers for Windows): you normally don't need to enable and enumerate all devices at boot time, just a basic display, one keyboard, and the disks required for booting the system, their SATA and USB controlers (the various "*HCI"), and a network adapter (Ethernet or Wifi) for your primary internet connection (or LAN connection if you're in a domain). Moderns BIOSes for EFI actually do not disable devices, they just preconfigure the BIOS without initializing these devices (that will be intialized later by the OS using its own drivers after boot), you configure just the minimum that allows the OS to be loaded and then the modern OS will no longer use the BIOS and will use its own drivers instead.
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