I assume you are referring to the ATX to AT power converters that allow and ATX power supply to be used on an AT motherboard.
Since the ATX and AT wiring standards are fixed and do not change those converters will work with any AT motherboard. If there was an alternate AT power standard then there would be a concern but this is not the case.
I will add one other concern that you may not be aware of. I have read that you need a dummy load on the +3.3V to make the power stable for the AT motherboard. Not all converters have this dummy load so I would recommend getting one that has it.
Here is an eBay auction that has what I am referring to as an example:
http://www.ebay.com/itm/261237563468?_trksid= … K%3AMEBIDX%3AIT
It's also a good idea to use ATX power supplies with over voltage protection because from what I understand these old AT motherboards do not have the hardware on them to manage this so you risk killing the motherboard by using questionable quality power supplies. I basically get ATX power supplies that are at least 80 Plus Bronze rated with over voltage protection for all my AT motherboards.
Roland MT-32 (old), CM-32LN, SC-55, SC-88VL, MT-120, SD-35, SD-20, SD-80, SD-90
Yamaha TG100, TG300, MDF2, MU15, MU100, MU2000EX + PLG150-DR + PLG150-PF + PLG150-VL
KORG NS5R, X5DR
AKAI SG01k
KAWAI GMega
KETRON SD2