first is correct. Second, with a pair of commas, would be correct but with a different meaning. "Your life, as you know, will be changed forever." This means that you are aware (know) that some event is about to change your life forever. The first sentence means that the way you know your life to be is about to change.

Hello! They are both correct - it just depends on the context of the sentences. #1 means that your life from this point on, will be different to what it was before. #2 means that your life will be changed forever and that you are aware of this fact - so (in my opinion) it should be punctuated as follows: Your life, as you know, will be changed forever.

1. is good. 2. Your life, as you know, will be changed forever. makes sense, but you can just say, 'Your life will be changed forever'.