Depends on your needs...
If I'm reading your post correctly, you are keeping the "if it ain't broke, don't fix it" option open, and that is comforting, especially if you still have clients depending on this box.
So that leads me to some questions: who's still being served by this unit, and for what services? If it's retired, might it be needed in service again if something else fails? If no one needs it now, what functions can you imagine adding if it's reconfigured?
Have you considered this possibility? Remove the disk drive, label it and put it on the shelf. Then install a scratch drive and play with distros and server roles to your heart's content.
Also, if you opt to keep it in service, and the hardware is getting older, what redundancy have you arranged to respond to catastrophic failure?