If you have multiple drives, you can copy everything onto second hdd and then either try the above link or do a clean install onto hdd using USB, after setting BIOS to UEFI boot methods, wipe all the partitions and start fresh.
Clean install is simplest way, just depends if you have the ability to backup drive before wiping it, as without using mbr2gpt, there is no way to convert a single drive to GPT without losing files. Outside of 3rd party apps that can do same thing.
Depending on your definition of "keep". You can always get a spare USB drive, SATA drive (be it HDD or SSD) and clone the whole thing, so you got a copy of every file. So yes you can "keep" them.
But if you intend to "keep" using things as they are without doing any of the extra work to prep, backup, clone, etc. You are asking to get something for nothing. That is NOT going to happen, you run the real risk of losing everything.
If he has a 2nd hdd, its easier to just copy all the files he wants off it, without cloning, and then fresh install win 10 on the other drive, knowing its all copied off. Then no need to run mbr2gpt either. No need for a backup of a pre converted hdd really.