How do I switch an MBR Debian install to GPT and UEFI without reinstalling?

This is more than just changing the partition table. The usual route is to boot a live system, convert the disk from MBR to GPT, create and format an EFI System Partition, then enter the existing installation with chroot and install the EFI version of GRUB.

The details depend on your partition layout, especially whether there is suitable space for the EFI partition. Check that before following a guide, and don't assume a guide that stops after the GPT conversion has finished the boot setup. Boot the live USB in UEFI mode so the firmware boot entry can be configured.

Back up more than just your home directory before changing partitions. A full disk backup gives you a way back if the conversion or bootloader setup goes wrong. Make sure you can restore it, not just that the backup file exists.

Have the live USB ready for repair too. A detected disk isn't necessarily bootable in UEFI mode until the EFI partition and bootloader are set up.

If you're not comfortable modifying the partition table and repairing the bootloader, backing up the drive and doing a fresh install is a reasonable alternative. Converting in place is possible, but it has several steps and isn't guaranteed to be the easiest recovery path.

The guides may look different while covering the same basic process. Compare them against the full sequence: GPT conversion, an EFI System Partition, and an EFI bootloader installed into the existing system. Older guides can still be useful, but check commands and package names against your Debian version. Plan an update of the distribution separately from the disk conversion.