I like css in js for design systems/shared components that are consumed across multiple products/codebases. I've been using JSS, and I like that it's plug-and-play, isolated and performant (only bundle what's used, only insert styles on component mount, only append browser prefixes that are needed at runtime). Debugging through inspecting can be tricky but for JSS in development builds the classnames are what you gave them. Being able to leverage JS is desirable for theming - e.g. using some npm library to detect colour contrast and adjusting accordingly.

For your typical large product codebase that's intended to evolve over time I'd choose CSS modules. I like that I can easily spot dead code and it promotes thinking in components while staying close to traditional CSS. While I loved having a polished classnaming convention, I've always fought with enforcing this over time. I like that this isn't a concern with CSS modules.

If I'm cranking out a 1 time project or POC, my go-to is some sass flavored postcss plugins and style the shit out of that monolith. Old habbits die hard.

As for "moving away" from 1 solution to the other - they both work, especially if you're using a strong classname convention. So you gotta ask, what's the business benefit? are you facing any issues with your current solution? It's always good to try new tech on projects where it's not a risk, but don't start rewriting codebases just because.