Originally Posted by AdamDZ

You can also move the hanging/locking mechanism on the bags or panniers to move them a couple of inches back forward which will move the panniers a couple of inches back. It's usually not very hard to do, and doesn't affect the balance of the pannier much, they're just bolted to the back of the pannier. You will need to drill new holes in the hard back and perhaps cut a seam or two to gain access to the bolts but it can be done.