This is a rather touchy topic given she has been in the news of late.
From what I heard, the show got rid of her because Paramount/UPN weren't willing to expand the budget (I presume they did a little, assuming Ryan was paid more than Lien), so someone had to go. First it was Wang, but a certain magazine saved his hide, so next up was Lien. Really, the only other potential fodder would be Neelix, and given how central the mess hall is (ship cafeteria) to scenes, it was unlikely he could be cut. Kes was just second fiddle in Sickbay and the cargo bay set was being replaced with Seven's Borgified cargo bay. If they got rid of Wang, I'm not sure who they planned to be an Ops. Random yellowshirt? There's no conspiracy, just new castmember + hard cap on the number of castmembers. And evidently it wasn't handled like DS9, where they could load up on supporting characters, some of whom appeared more than billed actors (i.e. Lofton).
As for her change, she was a minor actor who was out of the limelight for many years. For a lot of actors, one show is their only time in the spotlight and after that, they go back to obscurity. Look at Gabrielle from Xena for example (Renee O'Connor). Sidekick in every (?) episode of the top syndicated series for several years. And after that? Look at some of the movies she was in. Didn't star in much tv since. Even a show like Voyager, highest-rated show on UPN every show it was on (behind Smackdown, which I guess qualifies as a scripted show) didn't exactly send its cast onto great success (only Jeri Ryan seems to have continued her career, at least onto 2 series in the '00s). Heck, a lot of main actors disappear. The guy in Pretender? The guy in Highlander? How about some of the cast of Buffy, Angel, and Dawson's Creek, though WB was better at churning out actors able to get more gigs (Geller, Hannigan, Boreanz or however its spelled, Holmes). If you look closely, you'll notice a lot of castmembers of shows just fade away into the background.
