The problem with any "realistic" theory on how the Changelings could morph chemically or biologically is that it desperately needs a dose of magic to explain why Changelings also change their mass (both inertial and gravitational) when changing shape and size.
Let's remember that while the unconscious humanoid Odo in "Vortex" was "heavier than [he] look[s]" when the cretin Croden had to drag him around, the conscious drinking glass Odo in the very same episode was so light that Rom noticed nothing amiss when lifting him off a table on a tray! Similarly, Odo can be a seagull and fly on seagull wings, yet also throw his humanoid weight around in wrestling moves in one and the same scene
in "Homefront". Biology would be hard pressed to explain the sudden appearance or disappearance of weight and inertia.
Of course, the mass manipulation might be something exotic that's thrown in atop a basically biological or chemical process of shapeshifting. But if that much magic is accepted, then it's sort of redundant to avoid magic in the part that explains the shapeshifting.