Well, I think one can "explain" this from the same point of view that one can "explain" Odo being knocked out on occasion. ("Explain" in quotes because obviously what the changelings do is not very plausible.) But, based on what we see on DS9, what a changeling does has to be more than just altering physical form and appearance. Odo actually sees with his eyes, for example, in humanoid form, while in his natural state he must "see" with his whole body, in essence. So, that would be similar to comparing the link to humanoid sex. One good thing about the changelings are that they are an exception to the general rule of Trek forehead aliens. There are things about them that are actually alien. Unfortunately, the writers don't always do a good job of really exploiting that. Sometimes, though. Yeah, that whole experience suggests that a changeling can actually *become* humanoid. Though possibly this can only be accomplished with the aid of the link.