After reading my question if there's an obvious answer in a later episode, then just let me know and I'll delete my question.
QUARK: A Yridian I've been dealing with sold me something that might interest you.
ODO: I don't think so.
QUARK: You don't even know what it is.
ODO: I know I don't want it.
QUARK: In that case, can you tell me how to get in touch with the Founders? I know they'll want it.
ODO: What are you talking about? (Quark takes out an elaborate bottle of blue goo.)
QUARK: It's a changeling. Or it was, anyway. Since it's dead I'll let you have it for five slips of latinum.
There's no explanation of where the people who sold it to Quark got it from.
ODO: It's not going anywhere, Doctor. It doesn't know how. It's just a baby.
(After the opening credits, the blue goo is on top of a pulsing devise, and just a bit is now orange.)
SISKO: A baby changeling?
ODO: Centuries ago, my people sent a hundred of us out into the galaxy so we could learn about other races. When I was found, I looked very much like this.
So Odo implies that this was a custom done in the past, centuries ago. I'm not sure how the life cycle of a changeling works, but I assume if it's a "baby", it's not centuries old.
SISKO: Why would the Founders send such helpless creatures out into space?
ODO: To find out if the species they encountered posed any threat. What better way to gauge another race than to see how it treats the weak and vulnerable?
So Odo answers that it was to test other cultures, but again it was something that was practiced centuries ago.
ODO: Now if I were you, I'd hold this shape. Otherwise you'll be in for a little shock.
(Odo lifts off the cylinder and baby relaxes.)
ODO: Wait a minute.
(Baby goes back to being a cylinder.)
So clearly we see that it can change shapes on its own.
COMPUTER: The lifeform being monitored is displaying biomimetic fluctuations beyond stated parameters.
ODO: Have Doctor Mora meet me in the science lab.
Although Dr. Bashir earlier said the baby was stable, now it all of a sudden is not, and begins to deteriorate fast.
(Mora hands Odo the baby's cylinder. It's deep green now. Odo pours it out into his hand.)
ODO: Please, don't die. There's so much I want to show you. I was going to teach you how to become a Tarkalean hawk, remember?
(Baby soaks into Odo's hands.)
MORA: What happened? Where did it go?
BASHIR: It somehow integrated itself into Odo's body (Odo staggers.)
And now it gets absorbed into Odo's body and gives Odo his shape shifting power back again.
From an earlier episode DS9 "Things Past":
BASHIR: Well, it seems you're not as solid as you think. When I ran a neurochemical scan of your brain, I found residual traces of morphogenic enzymes which I've only detected before in changelings.
ODO: When the plasma storm hit the runabout it activated the enzymes and initiated a telepathic response.
So we see from this episode that Odo still has something of his changeling past still inside of him after the founders took his powers away.
Now that I've framed the question I'd like to be specific about what I'm asking.
Is the baby Changeling actually a changeling or was it something else sent by the Changelings to give Odo his power back? If it's just some kind of 'jump start' to give Odo his powers back then just tell me how you know that and that's good enough. If it's actually another changeling how does it actually transform Odo entirely, giving him his changeling "matrix" back, being that it was sick and dying, was much smaller, and was undoing what an entire lake of changelings did to Odo.