Maybe I'm not appreciating the Founders as I should, but literally creating organs and such inside of Odo, changing his DNA, etc., seems to be Q-level ability. Does the canon ever explain how Odo was changed? Or was he simply deceived into thinking that he was truly a human?
It's certainly not a deception. Dr Bashir scans him thoroughly and determines that not only does he have a full set of human internal organs, he also possesses blood and cells that read as human.
Bashir : I'm reading a heart, lungs, digestive system. It's as if he were human...
...It's blood, all right. Not a trace of changeling protoplasm in the entire sample... Type "O" negative, if you were wondering.
Later on we see him eating and drinking and suffering from several human ailments like tiredness and back pain.
Quite how they accomplished this transformation is never made explicitly clear. The changelings obviously possess the ability to completely mimic humanoid lifeforms (as evidenced by their impersonation of various Federation and Klingon individuals) and they have had ample time to both scan and collect DNA samples from humans. We know that they're specialists in genetic manipulation from their work on the Jem'Hadar, the Vorta and the Tosk.
The ultimate cure is that Odo is exposed to another changeling's "mutagenic proplasm" cells but again, this doesn't really shed any further light on the mechanism involved in his initial change.
In the case of the Founders, no Q-level abilities would be required to alter one of their own into a flesh and blood individual. It would be the easiest of transformation because of their naturally mutable cellular nature.
Think of it as a form of morphogenic encoding. The Founders locked his morphogenic enzymes into a Human shape and form, complete with all the benefits and disadvantages of the Human body.
Then they removed his ability to interact with his cellular structure. He was for all intents and purposes completely human and dependent on his Human physiology for survival.
Changelings were at least partially composed of morphogenic enzymes, the molecules responsible for their shapeshifting ability. (DS9: "Things Past") Common characteristics of Changelings included their biomolecular structure and morphogenic matrix.
In its natural state, a Changeling's body was a formless gelatinous mass, to which it had to revert every eighteen hours (Odo's example) in order to regenerate. (DS9: "The Forsaken", "The Storyteller", "The Alternate", "In Purgatory's Shadow"). If a Changeling remained in solid form for longer than sixteen hours, their body began to deteriorate and peel or "flake' away.
A morphogenic matrix was the cellular and quantum structure of a changeling, which allowed them to assume various shapes, sizes, weights, and forms.
The Founders cellular protoplasm is highly mutable, responsive to the control of a Founder sufficiently for Founder to take on the physical appearance, down to the cellular level of any organism it has morphogenic information for.
This level of mimicry sophistication is so great, any member of their species could be turned into a complete representative of a living organism or any inorganic substance they wished.
How They Did It
Changelings could take virtually any corporeal form, ranging from humanoid aliens to fog to reflective surfaces. They did not seem to be able to exist as or change to forms of energy. (DS9: "Chimera", "The Way of the Warrior")
They, or at least the Founders, also had the ability to closely mimic humanoid personalities to the point where even close friends and family often could not distinguish them from the originals. (DS9: "By Inferno's Light", "Apocalypse Rising", "Paradise Lost")
They could transform parts of themselves into smaller electric devices, like a combadge and make it operational, and were also capable of at least mimicking the shape and operation of a computer. Changelings could also mimic the form of fire and clouds.
- The control over a single organism who is a member of the Great Link is so complete, they are able to lock the genetic structure of any individual of the Link preventing them from having access to the control systems normally associated with that process. The equivalent of blinding their cells to any form of interaction from the individual.
- In 2372, the Founders forced Odo to return to the Great Link (on the new Founders' homeworld) for judgment after he killed a fellow Changeling. As punishment for his crime, Odo's shapeshifting abilities were taken away, and he was given Human form. (DS9: "Broken Link")
- I suspect this requires quite a bit of manipulation on the part of the Founders in the Great Link thus Odo had to return to the Link in order for them to lock Odo's morphogenic matrix beyond his individual control. It was only the introduction of new morphogenic material that gave Odo an awareness of his own locked-down state and restored his abilities.
This was no deception; Odo was, or at least registered, as human in likely every sense of the word.
While Odo wasn't the best shape shifter, we know that the Founders were considerably more skilled than he was. In "The Adversary" we are told that a Founder assuming human form will register as human to at least a tricorder.
EDDINGTON: I wish the internal sensors were online. It'd make it a lot easier to track down the changeling.
ODO: I'm not sure the sensors would help. If you scan me when I'm a rock, you'd detect a rock. I may not be able to duplicate a humanoid perfectly, but I'm guessing he can.
BASHIR: We scanned Ambassador Krajensky for tetryon particles. He registered as human.