I'd call the new testament more a shift in policy as to how you can get to heaven. It does not entirely invalidate the old testament. The prophecies are still relevant as quite a few of them are said to be fulfilled in the new testament, the old testament is seen as a perfectly valid historical document and so on. What changes is that the old testament says you have to sacrifice sheep in order to have your sins forgiven while Jesus says you can only get to heaven if you accept him as your lord and saviour and he will forgive your sins. Therein lies the big change. The old testament is not imperfect, it just contains some outdated rules of behavior that a christian would/should not follow anymore. Take this example where Jesus more or less seems to say that doing some work on a sabbath is okay if you're hungry for example:
http://www.biblestudytools.com/comme...e-sabbath.html
The jewish rules said the quiet on sabbath has to be obeyed but Jesus says the quiet was made so that people can rest, to benefit the people, not to force people to do nothing. He does not invalidate the old law, he just gives an interpretation of the spirit of the law that changes the way in which the old law should be applied. There's a change, some invalidation, some reinterpretation but not a complete invalidation, even some validation of prophecies as I said, Jesus is supposed to be the messiah who was prophesied in the old testament after all (jews would disagree of course).
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