Actually they are really very simple to change over... but you must first try to get TL494 AX supplies to work on... if not then ATX will do but 494 systems seem the easiest to understand.
The 12v and 5v rails are from the same single winding. The 5v is just a tap on the 12v winding, so control one, you control both... they do have different "in hand" strings... ie the 0-5v may be 4 in hand then tap .... then the 5-12v may be only 2 in hand... but the same winding none the less... hence different current for 12v and 5v circuits.
You get rid of everything on the LV side of the 494 except the timing cap ( pin5) and the timing resistor (pin 6) and get rid of anything to do with the dead time control ( pin 4)
Strip the board of everything else on the LV side, but for the totem pole transistors for driving the driver transformer, and any support resistors and diodes.... ie the driving side is left intact.
At this point you need only provide a voltage divider from the 5 or 12v rectifier for your voltage level ( I use pins 1 and 2 for this), and another divider for the current (I use pins 15 and 16), some feedback to pin3 to rate the gain..
Thats about 10 resistors and maybe a cap or two to reuse the soft start functions.
For this we end up with the ultimate battery charger. It can be used to dial up between 0 and 18A, can run from 5-28v or so.
It can drive dead loads ( DC motors under heavy start up loads )
It will be the cheapest most useful device you will ever make. It is a laboratory supply with complete short and current limit protection, .01v control.
If you experiment with power electronics, you really really NEED this so that you can screw anything you like up, and the current protection will protect your project from shorts or anything else, and provide variable voltages to test the unit under different conditions......
"Are there much better ways? Absolutely."..... I have never seen them. No other charger I have seen can do what a PC supply can be made to do.
It requires no money, no outlay, all the bits you need are there... but you will need to study the tl494 in detail and understand how it works to be confident of results.
Personally, I've been playing with electronics for approaching 30 years now, and wouldn't mess with it myself, but that's just me. YMMV.
"... geez Steve, you really need to do one before you depart the planet,... just to see what your missing out on.
I must have done 30 or more for different application for different people... takes an hour or so once you get the hang of it.... ATX needs a little more understanding to get the feel for the AUX supply built into it. Put a current meter on the front, with voltage and current control knobs, and this thing is indispensable on a electronics work bench.
They are a good bed ready for exploitation. If you want different voltages like 50 or 100, then wind another transformer. Getting the original ones apart requires heat from a gun or boiling water.... easier to get another bigger core, you provide the dividers, protect the 494 from over 34v, and you can have anything you want.