The Canon Answer:
Not given.
The Non-Canon Answer:
Well, take your pick:
- There are different races of Trill
humanoids. Some are spotted, some have weird heads.
- Surgical manipulation was "in" on
the Trill homeworld. By design or coincidence, both old and new Odan
had the same kind of "face job."
- There are two separate alien cultures
that have a symbiotic relationship with sluglike creatures, and coincidentally,
both call themselves the Trill.
The Lowdown:
The Trill underwent a number of conceptual changes between "The Host" and Dax's introduction in DS9:
1. Appearance - Odan from "The Host" was your typical Trek bumpy-headed alien; Dax sports a spotted look more akin to Kamala the Kriosian metamorph. Storywise one can give explanations similar to the Klingon smooth/bumpy argument--different subraces of Trill humanoids, cosmetic surgery, etc. In terms of production, someone apparently decided that the original Trill design didn't show off Terry Farrell's good looks well enough, so they tossed continuity aside in favor of a more appealing design (you can see footage of Terry Farrell in the original design as well as other, well, "test patterns" in the Making of Deep Space Nine video that was released in '93).
2. Personality - this change was a little more subtle and implicit, but in "The Host" I got the impression that the symbiont's personality remained unchanged from host to host--that the humanoid was essentially an empty shell that the symbiont hitched a ride on. DS9 introduced the concept that a joined Trill is the blending of the host's personality with the symbiont's, and got a lot of mileage out of the "Trill angst" and "just how much of you is the old you" stuff out of that, things that were apparently non-issues with Odan.
3. Secrecy - was the Trill practice of Joining a closely-guarded secret among the Trill and unknown to the Federation prior to "The Host"? The episode certainly made it seem like it was news to Picard and Company. Doesn't really make the Federation look good--shouldn't they know this sort of stuff before admitting a culture into the Federation? In DS9, the impression was that this is common knowledge and has been for a long time.
Odan's unwillingness to use the transporter
was supposed to be a clue that would lead us to the surprise in "The Host"
about the nature of the Trill, which goes toward that "secrecy" thing.
With DS9, that whole part of the Trill idea was forgotten. One can try
to explain away Odan's insistence on not being transported on personal
preference, or maybe a rare condition with the symbiont that precludes
transport (but isn't prevalent among joined Trills). But in reality, you
can chalk it up to the DS9 creators only taking the bare minimum of the
Trill concept--the joined species thing--and building on it, ignoring things
that were established in "The Host."