Its usually much easier to go from a smaller case to a larger case than to go from a large case to a small

One thing most OEM SFF setup do not use standard components for mother board and power supplies it should not be too hard to mount the mother board just need to setup new mounting hold offs. However changing the power supply might be a little harder another option would be to just replace the power supply with a standard atx power supply which should mount in the new case without any problems.