The Server was just a configuration, not a real hardware difference with the non-server MacPro.
It would have shipped with OS X Server system software, and came with two hard drives as standard.
Other than that, both would have the same ECC memory installed, and video cards might have included different models. Not any real differencs in the hardware.
You can just erase a server system, and reinstall a "normal" non-server macOS system.