It all starts with what you really want: do you want to stick to coding or do you like design better?
I've actually transitioned a bit as well, but it came more naturally then I expected. I've been making HTML/CSS/JS since the beginning of my career and design has always been an interest of mine. However, after doing this for 15 years (Photoshop > HTML/CSS/JS), I felt like I needed to widen my skills and perspective, so I've found a job as UI designer as well (I use the term Front End Designer). Basically I think of a design in my head and code that directly in HTML/CSS. No Photoshop needed, plus I get a working prototype directly.
But like I said: It's all about what you want mostly. And it depends on how long you've been doing this for. If you just started your career, I'd say stick with it for now. Learn all the design stuff on the side and if you really want to become a UI Designer, you can find a job like that. :)
On the money side of things: real designers who just do Photoshop/Illustrator do get paid less, but a Front End Developer who can design... that's pretty new in this field I think, so you might get paid as a developer. For me, I get paid along the lines of a regular Front End Developer, so no decrease in pay for me. :)
And take all the points of @bmlyon in consideration.
The worst thing that could happen is that a UI designer position is not for you after all, but then you could get back into a Ruby job anyway. It's not bad to give it a go.
I've actually tried to be a project manager for two months. It was interesting as hell, working for one of the top design bureaus in the Netherlands, but it just wasn't for me (mostly due to lack of experience). It didn't work out, found a new job within a month.
Developers are in high demand now anyway, so the risk is quite low.
Good luck with deciding! :)