Is it not possible that these changes are intentional? That entering middle age, she doesn't want to blast away as she did as Violet (or the mother in TOMMY, when I first saw her; she understudied and went on to fabulous effect).
I've seen all of her Broadway appearances, several times in SIDESHOW. But don't you recall how many people disliked the SIDESHOW sound? I recall even fans of belting saying the two women sounded like they were "yelling..." The SIDESHOW vocals, discussed here as her golden moment, were strikingly theatrical and unique -- designed to serve the Krieger blend of pastiche (the period 'performance' duets) and pop power ballads, which took the roof off the 46th Street. Many people felt the pop-rock influence actually damaged SIDESHOW's credibiliy as story, that the Daisy and Violet of the 1920s had been given an early Lloyd Webber treatment when they deserved, say, Irving Berlin. I disagreed, the goal was to explore their sexuality and yearning, but it's possible that Ripley doesn't want to be boxed in as a screechy pop-belter, particularly in the IDOL era of post-Whitney, post Maria. SIDESHOW was over a decade ago.
"I'm a comedian, but in my spare time, things bother me." Garry Shandling