I haven't switched entirely to yoga, but I have switched entirely to all low impact, fusionish, flowy workouts (with yoga every other day) from all high impact cardio and heavy weights. I hit 40 this year, and my body just gave out after 20 years of high impact stuff with little stretching or breaks. I had gained 10 pounds in the past year and just felt burned out.
Thanks to everyone's suggestions here, I gave up the fear of switching and just started listening to my body instead of my heart rate monitor. It's been 4 months now -- I've lost those 10 pounds, my flexibility has never been better, my knees don't hurt, my body feels good, I look forward to my workouts, I sleep better, and I just don't feel like I need to eat as much anymore. I don't I'll ever return to high impact stuff or even much cardio at all anymore.
I don't think I've lost much cardio capacity at all actually -- I had signed up for a 10k run earlier in the year, but since I've switched to all low impact, I figured I'd just walk it. Well, I felt great that day, so I started jogging it and figured I'd walk when I got tired. But I ended up jogging the whole thing! And my time was only 4 minutes slower than a few years ago when I was training pretty heavily and killing myself!