reuse or use new concrette?
 

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?

How old are the post? If there over 10 years old I'd replace them, Old concrete does not like to stick to new.

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.