Actually the question is age old. As a fence contractor for over thirty years We daily run into this question from homeowners. " We just replaced the fence a few years ago and the fence boards are perfect, can't we just replace the posts and re-use the fence boards and rails?" No because when you take the fence apart the boards split and crack. Yes if you spent many hours you can but the time outweighs the cost of just building a new fence.
Now for years and years I have been seeing the same answers on dozens of forums that are totally incorrect and it baffles me. The same answers just reworded from writers that do not do good research.
Post Rot can not occur underground. Protecting the post at the base is what needs to be done. Two of the four elements which rot needs to live- proliferate and exist can not physically happen underground. One is oxygen and the other is fuel- microbes and fungus. http://ucanr.edu/sites/forestry/files/172688.pdf
Rot starts at the base of the post where soil comes in contact with the wood. Moisture and fuel rot the wood at that place where the post exit the earth, as a homeowner think and you will see where the posts break and snap... Now we pull posts out daily and the bottom of the posts still often have the tags on them from the lumber yards. http://thepostcollar.com/main_page.html
Yes one thing is accurate. The concrete needs to be crowned (sloped) away from the post so water and soil do not touch the posts. But there is also a problem there. In time with wind and mostly because a contractor or homeowner doesnt seal the concrete properly, water will seep in the tiny gap between the post and concrete and sit in there like a pool. In the long run doing more damage- constant moisture and oxygen and fuel. So the posts rot even faster. The base of the post need to be protected. There is a new product on the market called The Post Collar that solves that problem. Or be certain when you crown the concrete that you actually float the concrete away and form a tight seal. There is only one product on the market that does this, The Post Collar. We are using them on all our fences now. http://www.losgatosfence.com/home.html
I am not taking the time to do this to advertise my company. Just taking the time on a Sunday morning to try and reeducate people and writers and to answer this age old question asked and respond to in this article.