Like avr says, there's ways to change armor's size, but nothing that I know of off-hand that turns armor into barding. The fitting property (+1,000 gp) will change an armor's size, but doesn't say that it otherwise drastically alters an armor to suit a creature's body-type (other than size). It will not have three or four arm holes if you have those features. Nor will it close up the chest hole on a magical suit of fitting armor that was made for a Large-sized Athach (a giant with a third arm coming out of its chest). It might shrink to your size on a human, but there will be a hole custom-made right in the chest (as for how that affects your AC when wearing it... that's all a GM's call, but usually a hole right in the chest isn't good for human armor, Athach's not so bothered).
In regards to centaurs, they have no special restrictions on armor, but that's not to say that there aren't obvious equipment restrictions pointed out. Their torsos are clearly defined as being human-sized (medium), despite their Large-sized bodies. They also aren't necessarily two actually different creature parts 'stuck together'. For instance, there's no indication that a heart is in the human chest and the horse body, or that there's multiple lungs in both. If you stab a centaur through the heart in their human chest, it hasn't really been established that there's a second one in the horse body. So it's common sense that the armor they wear is likely humanoid enough to provide suitable protection. Ie. Their head, hearts, kidneys, lungs, etc are in the parts that are protected by a breastplate. That leaves lots of mass protected only by their hide (or what they want to cover it with), but that's enough to cover the vital bits enough for AC purposes.
(Not saying it's perfect. For instance, could the stomach in a human torso comfortably hold the amount of food a creature that size needs? Probably not... maybe it has its stomach in the horse part where a horse's would be... maybe it has more than one... I'm just saying it's not really well-pointed out.)
The problem with letting you just handwave armor into barding is that it is not the same thing. Barding is more expensive than armor and has different effects because it's called barding, ie. flying mounts can't fly in medium barding, something flying humanoids don't necessarily have in medium armor.
Large barding is 4x the cost of similar medium armor. I don't know what size armor your half-plate is (in case you took it off a large giant), but even if it was large size, it would still cost 2x as much as barding. There is a balancing reason for it. While market costs and gold piece availability and character wealth at levels is debatable, that doesn't negate that there is a not-insignificant difference in the two items in their costs and properties.
Allowing the transfer of such armor for such a low enchantment cost as fitting seems a bit of a stretch (but I'm not saying another method might not exist... they've made wackier things). Your best bet might just be to sell the armor or start saving up or... your GM might allow it even if you can't find an official way (but he should still consider why there isn't one first).
