problem publishing WP to an existing domain name

I have about 70 accounts who already have websites that I\’m working on upgrading to mobile responsive design over the next 3 months. I HAVE A GIGANTIC PROBLEM THOUGH….All the info I find regarding changing root directory etc…(because of their rankings I have to use the same page URLs of course), so I have to change those to be the exact same link in WordPress…but the real issue arises when I try to publish my new WordPress site to an existing domain name. I can\’t imagine anything being so hard to do actually. I\’m used to hitting a button to publish (super fast and simple)…but this is very long and drawn-out process. I\’ve searched online for step-by-step instruction to do it more efficiently, but haven\’t really come-across the right set of directions that get my there in any less than 3 hours. The majority of my client sites aren\’t large – like 3 to 6 pages. Please can you to help me with this. Even my tech partner is having trouble getting past this one area. And I really don\’t know this part of the design aspect…..like I said, nothing should be that difficult to do and if I was publishing a brand new site – it might be a breeze, but publishing to an already existing site with it\’s own page URLs already created is practically impossible. I don\’t even know what I did to get that sample site above published – obviously something right – but ever since then, the rest are super – troublesome, time consuming and I even lost a complete site 2 nights ago, had to rebuild it finally after much trial and error, he was able to get it published.