I have an older 4' fence in my backyard that is falling apart. I plan on tearing it completely out and replacing with a 6' privacy fence. Should I dig out the concreted old posts and use the same holes or would they be too big from digging out the old posts? I could fill them all in and redig the new holes... what is common practice? This is the first fence I will have build so its all new to me. Also, what is the best practice to following a slight grade on my property? Should I follow the lay of the land or keep panels level and drop each panel to resemble something like 8' stair steps?