Specialized Sunrise Scanner PSU AT/ATX PSU Help

Hi, My question is that of a AT Switching Power Supply. I have a very special industrial Sunrise Microfilm Scanner. The unit is very old and uses a P2 Mobo and ISA cards. It has a lamp which is also a 400w psu "custom Made" and it connects to a AT Switching PSU and another PSU which is an emi filter of some type.

The problem is this. It is an AT psu and an AT mobo. I want to upgrade the mobo to an ATX Mobo but I kind of understand that an atx psu uses an electronic switch and an AT psu is different. As you know, an AT psu has a special switch with 4 wires, 2 positive and 2 negative. Well the switch is taken off and is connected directly to the lamp psu. This acts as the power switch for the entire machine. The computer is turned on via a switch connected to the lamp.

So to get this straight. The psu switch is cut off and spliced to a 2 cable connector that plugs directly into the lamp which is also a power supply and the switch for the power is powered from the lamp. I want to upgrade this AT psu to an ATX psu but will not be able to connect the switch to the lamp because I believe some type of voltage passes through the switch but am not sure. The devices have to be powered simultaneously.

This question would require an answer from someone who is very familiar with the differences in at/atx switching and electrical voltages.

Also, I just remembered, an atx psu does not have an on off switch like an at one. hmm. Any help???