There are many potential benefits to replacing all your interior doors at the same time. First, many contractors may offer discounts for doing multiple doors in one job. Second, door retailers may offer discounts for bulk purchases of sets of doors. Lastly, having them all done at the same time can help ensure uniformity. If you decide to do half the doors now, and then go back 6 months later to do the other half, you might not have the same doors available for purchase. Also, it can make it easier for financing to tie it all together into one loan rather than splitting it up into several smaller loans.