It`s not worth the hassle replacing the PSU with an AT or ATX one if the original is still working.
Apart from the motherboard power connector, you`d have to figure out how to connect the power switch (built into the A4000 PSU, connected to the button by a long rod) and also cover the holes in the back of the case with the smaller PSU.
After I replaced the fan in mine, I also drilled a large hole in the side of the PSU nearest the motherboard and put a grill on it to stop wires getting in.
The IDE and SCSI cables to the drives at the front of the case almost blocked the vents in the front of the PSU, so no air could get in!
