Hooberus, God has changed the rules in the past and He still has that right. For the most part revelation will not refute past revelation, but it does not hold true always. God said in Exodus 20:13 "Thou shalt not kill" but twelve chapters later He commands (through His prophet) "slay every man his brother, and every man his companion". In Leviticus certain women are ordered to be killed. You have to know if a prophet is a true prophet or not before you can whether to follow orders that contradict scripture. How can you tell? The Bible says that his fruits will testify of the actuality of his calling. There is no other criteria for measuring a prophet sanctioned in the Bible.
Interpretation of scripture is also another factor that enters into this equation. Arguments can be solidly made for and against almost any interpretation of scripture; nonetheless, I will not use a prophet as a trump card here. You are free to open discussion.