Hello,

I'm not quite sure what you mean by 'Legacy Mode'. I'll assume your
talking about BIOS. If you want to use the installer to install
grub2-bios make sure to boot the installer disk in BIOS mode. Vice
versa to install grub2-EFI make sure to boot the disk in EFI mode.
Depending on your implementation of EFI you may need to disable
'SecureBoot' which prevents you from running "unauthorized" operating
systems such as Devuan.

If you want to install Devuan ontop of an existing Linux install you
can likely do this if your /home directory was installed as a separate
partition. Simply do the advanced install and setup your partitioning
manually to use the existing home directory. As with any potentially
destructive disk operation such as formatting or partitioning make sure
you have offline external backups of anything your not prepared to lose.