The original poster didn't ask if it is possible and how to rename a predefined folder, but how to change the registry defaults to different folders, by using a command at the command prompt level. I do not know if this is possible and I also don't understand why would anyone want to do this, this way. But, if possible, it won't affect windows functioning. I also did this by using the windows built-in tools and everything works fine.
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Oh thats an interesting idea. I also get what you meant, I typed meaning that I didn't know if changing the folder would automatically change the path already inputted into any game you're using or even the operating system. I can see renaming "My Documents" to "Mi Documents" would have games or installed programs still looking for "My Documents" instead of the updated name, thus losing functions of that program. I also couldn't easily test it as I was not at my computer.
For cmd:
you'd need to use reg add with the /f to overwrite it and also change the long word from hkey local.... etc... to HKCU,HKLM... etc. you can do reg add /? to see details on the command.
HKCU/rest/of/the/path /t REG_SZ /v value /d data you want change /f (to force the change)
reg_sz being a string, it could be another type like a multistring. I'll link the
microsoft docs on reg add.
the path for some of the folder options would be:
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders
i believe this should be potentially repeatable with group policy as well through the windows components section.
go onto regedit and follow the path to do it directly in regedit instead of cmd.
Other than that i'm admittedly not certain, still i hope it helps
if not, what do you specifically want command line to do for you here? Op you mention changing the defaults, the default whats?