reuse or use new concrette?
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.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.