on windows, it's a regedit change. open run dialog. then open regedit. then navigate to regedit directory:
HKEY_LOCAL_MACHINE\SOFTWARE\OpenToonz\OpenToonz\1.0
edit the variable: TOONZPROJECTS
Enter the full path to your new project root.
Restart OpenToonz and you will have a new projectroot.
You can also add multiple project roots into the same regedit variable. The variable is actually a list of paths delimited by ":".
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On mac the project root is stored in SystemVar.ini. I don't have a mac to test on tho.