I had a wooden fence that was blocking half of my backyard but i dug it out and removed the posts, the posts still have the concrette on them. Now this may be a stupid question and probably already know the answer but in order to put it back up i will have to remove the exciting concrette that is on it right?
I would break the old concrete off and check the posts for rot. You would just have to dig a bigger post hole to try to use the existing concrete collars and it will be hard to plumb and line up your posts with that mass around the base.
I used the dry method of setting post most of the time. Set your post. Add dry mix. Plumb and line the posts. Pour in water to cover.