Kira instead of Ro on DS9 was because Michelle Forbes turned down the offer. (And one of DS9's producers wrote the episode that introduced her in TNG so if he wanted he could have forgone the residuals.I had never heard about any of that before, it's scary how big of an impact residuals can have on shows. I know we got Paris instead of Locarno in Star Trek: Voyager because of residuals, and I think they might have also the reason we got Kira instead of Ro in Deep Space Nine.
You mentioned what happened with the Locarno/Paris situation (and in that case both characters were played by the same actor.)
The other Trek related instance was Enterprise as T'Pol was originally going to be 'T'Pau (from TOS S2 Amok Time) - and yes they eventually did show T'Pau in two episodes but they paid the royalties for those two episodes instead of the full run of 98.

But yeah, studios are always looking to keep what they consider unnecessary production costs down.

