As health care power of attorney, can I change my husband's living will?

I believe your decisions can over-ride the living will if there are conditions that weren't foreseen when the living will was written. This is common, as none of us know the details of what will happen in our lives. Living wills can't cover everything, which is one reason why we appoint a health POA. You may want to check with an estate attorney or elder law attorney if you want to do something dramatically different than what the living will states, but I do believe your POA is there for you to make the best decisions you can, knowing the person trusted you with this power. Take care. It sounds like you are going through a tough time.
Carol