How is this particular bit of mechanics in any way different from Summoned Armor?
The mechanics are not.
The difference is that Summoned Armor doesn't give a PC an instant disguise. Summoned Armor can be any armor because it's not as potentially game unbalancing as instant disguise can be.
Imposter's Armor needs a downside to be balanced. As written, the fact that the user would be using heavy armor is such a downside since the heavy armor users would have difficulty getting the proper skills to really take advantage of it (and they have the AC penalty). Light armor users would have difficulty of getting the proper armor feats to really take advantage of it.
By changing the rules for the armor, it gives a major advantage to the light armor users that the heavy armor users don't get.
Again, this was mostly for RP support of the most poorly supported PC race in the entire game. Mechanically, I can pull of what I want without this proposal. I just think that the necessary steps are silly.
If we make it a house rule, it isn't limited to the Changeling race. It gives all light armor users a way to have a better version of Summoned Armor at the exact same price.
While it is true that the Changeling has limited feat support, the ability to change one's entire appearance in an instant would be a huge RPing advantage.
Gaining the ability to change into any humanoid and any clothing is almost the equivalent of Invisibility at will.
To me, that's really potent.
A player who wants that and all of the disguise related skills (i.e. Bluff, Insight, Perception, Stealth) should have to pay for it. Either by being a heavy armor user and acquiring the appropriate skills, or by being a light armor user and acquiring the appropriate armor feats.
We shouldn't hand the harder to acquire portion of it (instant change of clothing) to him on a silver platter with a single relatively cheap magic item.
I guess a different option would be to propose a feat or something that allowed a Changeling to alter their clothes.
It's called a Hat of Disguise.
It might not be apparent, but it looks like the designers might have actually thought this one out.
To get near perfect shapeshifting (i.e. where appearance, armor, and equipment changes) is difficult and properly so.
Hat of Disguise can be foiled by tactile or audible senses.
Imposter's Armor loses AC and does nothing for other equipment.
Potion of Mimicry uses up a healing surge and is expensive for the higher bonuses and has the same disadvantages of a Hat of Disguise.
Face Stealing Ring is higher Paragon level and is also an illusion.