The 2nd amendment is an amendment to the Constitution. Which, by definition, is a change to it. Which is a demonstration that it can be changed. This is not simply a document that is held onto for 200 years and not criticized, revised, or changed.
That is to say, the reason the right to bear arms has been a part of the Constitution for 200 years is not simply because it was part of the Constitution 200 years ago. But because there has not been significant cause for that right to be revoked.