Alright Okay Here is my suggestions about changelings starting off with.
Make changelings be little parasite bugs that can dive at there target and burrow into said persons skin. It takes 2 minutes for them to get to the brain and merge, The human wouldn't know it untill they got tooken over unless they did notice it, You guys could add it so you could wrap cable coil around said limb the parasite digged into. They'd mature while trying to make it to the brain permamerging them to said limb, It'd make the host lose control of said limb.
. Changelings should start off with all skills if they merge with the brain succesfully.
They'd still be able to change there shape and form, They'd just start off as parasites. Very very tiny parasites, Hard to see with the untrained eye.