You can change, add or edit your shipping policies any time. I have five myself that I have set up though I usually only use two. If you go to your account under Business Policies, go to Shipping. In there you will see all policies ( not just shipping) but your shipping policies should be listed in there. When you change a policy it will ask you something like "change to existing listings" which you can do. Note that there is a shipping policy set as Default (not sure how to change the default) but in my case it seems to choose the method I use most often. When you're doing your listings up double check under shipping which one it is using. There is a dropdown where you can choose from any of the ones you have set up. When you use "sell similar" it picks up the one you used before but you can still change it if you wish.
Note I do my listings one at a time, I'm not sure how this works with bulk or the tools. This method worths consistently when you do them up one at a time (new) or will change the ones you have listed if you mark off (apply change to existing listings).
Hope this helps!
