This is one I can answer. Because my M1009 came to me as a 12v system. POORLY converted I might add - they ONLY swapped the started and two battery cables - not the other things that make a good 12v conversion work properly. Plus the county sheriff's department that had it hacked the wiring harness pretty badly as well.
So things didn't work so great. At least not ALL of the electrical stuff and not all the time.
My conversion consisted of swapping the 12v starter with a 24v starter (then I had to replace a bad solenoid - actually easy). Plus replace a couple of battery cables.
IT DOES make a difference. Mine used to be hard to start. But now, she fires with one turn of the key. I used to have to cycle it at least twice (three times in the winter, but I live in Denver). Now she fires right up. Even WITH snow on the ground and the temp well below freezing.
So it's doable.
Cheap? Not really. But I'm anal and like to go at things one step at a time.
Mike at OD Iron is your friend and has ALL the parts (extra alt and 24v starter) to help you on your way.
Others here will gladly step up and help as well.
I'm TICKLED that mine is now back to 24v.
You want an EASY way to check to see if it's 12v or not? Check you NATO slave plug. Chances are if it's been converted, your NATO slave plug wires have been clipped so NO ONE attaches a 24v power supply to your truck. Because IF it's been converted and you do attach a 24v power supply, you will definitely fry something.
My NATO slave plug wires are clipped and pulled to the front of the grill so you can see that they are clipped. So I've gotta replace those two cables as well.
Hope that gives you some insight.
Here's a couple of posts regarding my problems. And what I did to resolve them.
http://www.steelsoldiers.com/cucv/30979-cucv-alternators-im-still-confused.html
http://www.steelsoldiers.com/cucv/32194-huge-kudos-od-iron-2.html#post392529
http://www.steelsoldiers.com/cucv/35639-everything-working-great-then.html#post392527
I hope those links help.