I actually just upgraded my Dell Optiplex 990 SFF to a new full ATX case and had to get a new motherboard. Dell does some funky stuff to their motherboard to get it to fit into their custom cases. For instance the CPU cooler clipped into the case through the motherboard This was one of the ways the motherboard was secured into the case. The other mount points were entirely non-standard so no other case would fit. I did consider making some modifications to the case with a hacksaw before deciding to just get a new case and motherboard. The new Motherboard was pretty cheap because I was using a 4th-gen intel i3. Not great specs but good enough for a homebrew NAS.