Or then there wouldn't be a single recurring Trill slug character with many faces, but rather a group of slugs coming and going. The DS9 concept could have accommodated a constant flow of visiting characters whom the heroes are supposed to trust even though they only work with them for one or two episodes, and a certain percentage of these could have come with a tummy-hidden agenda. Sort of like the Founders, but less sinister.
I know some of you will go "No, it actually all fits perfectly, you see... [insert incredibly conveluted explanations here]" but a retcon is a retcon.
And Star Trek essentially consists of nothing but retcons. Why should there be something bad about that? There's no preplanning in this sort of television, there is only postplanning... But postplanning is what gives us backstories, continuity and plot twists, all of these sort of by definition.