I respectfully disagree that Perron was favored by Hitch, simply because he use to give him more minutes compared to Roy's current minutes. Sometimes you have to use the ingredients you have on hand. Once they realized that Schwartz and Tarasenko could handle more minutes, it was "see ya" to Perron. Perron just wasn't a fit for Hitch and I am guessing Team Backes (based on some of his past pointed comments during last season, for example suggesting that there were those worried more about stats than playing a team game).
As for Roy, he was obviously a back up plan and not a first choice, but they felt they needed a playmaking center. Not their optimal choice, but they wanted something since they desired more scoring, even if an imperfect fit. If I remember correctly, they went after at least top 6 centers before settling on Roy (Weiss, Flippula, Vinny).
With his demotion to wing, I think it is obvious he was not meeting Hitch's requirements as to a center's defensive abilities, so he promoted Sobotka as a stopgap. As to obtaining a center, at the moment, it appears that gamble is not paying off. With all that said, Roy has put up a respectable amount of points and seems on track to obtain his expected output. He paid has a supplementary piece, and so far he is providing that value, but not checking all the boxes for the Blues at this point in the season. If he racks up his points, he will probably parlay that result into a contract with another team.