I don't think that that is technically a plot hole, as Odo's morphing ability isn't really plot relevant. But, just because his future self has shared the memories of being able to better imitate humanoid features, it doesn't follow that younger Odo would have the ability suddenly.
Odo has linked with other changelings before, changelings who were able to perfectly imitate humanoids, and he never picked up the ability from them, either. It seems to be literally a matter of practice. If you shake hands with a body builder, you don't become muscular, do you? You actually have to work out yourself to get the muscles. I can only assume the same is true of Odo.
Not a good analogy, as it doesn't accurately depict how linking works. Changelings describe the Great Link as a "merging of thought and form," which implies that ideas, emotions, and memories are shared between both parties of the endeavor. Shaking hands with a body builder wouldn't allow one to become muscular, but sharing the thoughts of a body builder should provide one with ideas on how to duplicate the other person's achievements.