I don't know how well changelings can change form but a Hat of Disguise would be useful to a spy.
Ooh, clever! I like that.It doesn't matter- it would be useful to a Changeling as well.
Why?
Because, if it is discovered and then confiscated, the Changeling then "reverts" to a preselected appearance not his own...so when he alters his appearance later, nobody will be expecting it.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.