You can replace the battery on every model. What's different is how much work it takes. If the back panel is removable, then it's very easy to do it at home, the same way you would replace the batteries in a TV remote. But if the back panel isn't removable, then you should take it to a shop to have the battery replaced, unless you feel you are technical enough to do it at home.
No should be quiet cheap for the A6. Less than £50 Carphone Warehouse quotes £39 for a battery replacement of the A6. Other company prices may differ slightly but should be around the same.