Did you ever stop to wonder how the ancient yogis figured out the health benefits of the various asanas? Back before modern science, a lot of trial and error was probably involved. Over the years, yogis learned what seemed to provide the most benefit for the body by observing the effects of asana and pranayama. Today, we have even more information to help us understand what types of exercise helps the body the most.
For some, it may seem almost sacrilegious to change a yoga posture, but many yoga teachers are doing just that. Not only do we have modern science to help us assess the effects yoga has on the body, we use yoga in different ways than the ancient yogis of old did. Many of the top yoga teachers have backgrounds in health and kinesiology (the study of the body). As physical therapists and health professionals study yoga, they are bringing their own expertise to the yoga world.
So it's perhaps not surprising to find out that the "word on the yoga street" is that Triangle pose might be better performed with the back hip facing forward slightly, rather than trying to keep both hips in straight alignment as if pressed through panes of glass. The new modification seems to be kinder on the hips as well as the SI joint.
Warrior I is now often taught with the feet in "two tracks," that is, with the legs a little wider than traditionally taught. You don't need to have your front heel in line with your back arch - you can have the back foot out to the side a little. This also helps the hip joints.
Many other traditional yoga poses have alternative versions that can be done by people with more limited flexibility or health concerns. Props such as blocks and bolsters can make a huge difference between taking on a pose that's stressful for the body versus one that is supportive.
One of the benefits of doing yoga teacher training - even if you aren't planning on teaching yourself - is that you can learn an awful lot about how to be safe in yoga. You also get "insider" knowledge on the trends happening in yoga. Of course, what you'll learn depends on the school and the teacher, but if you do have a good teacher, you'll learn a lot of useful information to help you in your personal practice.
You don't need to commit to an entire yoga teacher training program - many yoga schools offer weekend workshops and other short courses that yoga teachers take for continuing education. Depending on the school, you will be able to take these courses without prior teacher training. You can also find many modifications to yoga postures in books and videos geared towards people who need a gentler yoga practice.
You'll still find that various yoga schools and teachers disagree on the best way to do poses, but that's where your own personal discernment and practice comes in. Ultimately, you'll find what works best for you by trying variations of asanas and modifications yourself. Listen to your body, and use your common sense as well.