First, I recommend being more precise in questions. "from a bootable USB" what is that referring to? A CD-Rom, A DVD-Rom, a USB-stick or a hard drive?
"changing legacy mode to EUFI mode and disabling secure boot"
I don't kow what "EUFI" means. I can only guess that it's an error.
You should take a little bit more care.
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Regarding your question here is my answer:
Checking the risk of a MBR to GPT conversion:
A conversion from MBR to GPT can be considered as low risk when the conversion does not require moving or shrinking of partitions.
The need to move or shrink partitions arises from the fact that the GPT structure reside in well defined locations.
Look at the image in
https://en.wikipedia.org/wiki/GUID_Partition_Table
The GPT structure requires the first 34 sectors of the disk (from sector 0 until 33) and the last 33 (because there is no backup of the protective MBR residing in sector 0).
If your partitions do not touch this area the GPT structure can be written as its just a defined list of pointers at the beginning and the end of the disk pointing to the beginning of the partitions inside of the disk.
If your partition starts in the supposed-to-be GPT area at the beginning of the disk it has to be moved forward into the direction to the end of the disk (towards higher LBA numbers). This moving operation is risky. Imagine what happens if you loose your power supply while doing that.
If moving makes the partition touch the last 33 sectors of the disk free space has to be created by moving one or a couple of partitions backwards (in the direction of sector number 0) assuming that there is still free space in between of the partitions or at least one partition has to be shrinked. If the partition to be shrinked is not the last one on the disk, moving is required as well. This process can also be considered as risky.
If the partitions inside do not touch the first 34 sectors you still will have to see if the partitions touch the last 33 sectors. This would require moving and/or shrinking to free those last 33 sectors.
Even when being in a low risk environment it is worth being paranoid and recording the current partition structure. I would recommend Testdisk for it. When going into drive analysis you have the possibility to backup the partition structure of the drive into a file labeled "backup.log" You can read that with a text editor.
Checking free space at the beginning of the disk
Having that information you can determine whether the operation is risky or not. The availability of free space at the beginning is visible directly when looking into "backup.log".
Checking free space at the end of the disk
To check the necessary free area at the end of the disk you either look at the disk housing where the number of sectors should be printed or you use a linux tool like hdparm.
Here is an example linked, what matters is the LBA48 figure on the picture:
https://www.geeksforgeeks.org/hdparm-command-in-linux-with-examples/
Calculate the last sector used of the last partition and check if there are still 33 free sectors remaining.
Conclusion
If the GPT area at the beginning of the disk or the end of the disk is occupied you are better of reinstalling a new disk instead of trying out a risky conversion.
All the above applies to a sector size of 512 bytes at the external side of the hard disk interface. This could be real 512 byte sectors or 512 byte emulated (typically internal 4096 bytes). In case of the harddisk in question has a sector size of 4096 bytes externally (1:1 pass through of internal sectors of 4096 bytes) the sector numbering changes.