I had 3 posts of my fence rot out and three sections needed repair. After having dug up the cement balls from underground a week ago, which was quite the ordeal, needing use of pry bars, shovels and a mini jackhammer. I had some help due to me being cautious (shoulder).
This past Saturday. My friend picked up a yard of blue stone. With large plastic tubes to encase the posts, big stones to surround it, then fill with blue stones, packed it down. Put up the fences. 8 am to 2pm . Non-stop physical job. I was able to dig and shovel stone without any problems. Good to have a friend who’s done this before around. 
Old fence. No need to use concrete. Besides it took over 40 years to rot out. I won’t be here 40 years from now.