There must be a way to transfer to wordpress

I was just told there is no way to do this so I thought I would ask you guys.

I have a couple of existing sites some I built in dreamweaver and some were set up on godaddys website tonight temporarily. They all have one domain and have been ranking for over a year. I want to create all new sites in wordpress in Atahualpa. However, it wont allow me to setup a new database with the corresponding domain because it already is attached to another. Some of these have their own accounts and some are under my unlimited server account. I was told the only way to do this without losing links and ranking is to turn off the old site, then build the new one under the domain. But I don't want that much downtime. There are several ways I thought to do this but all seem to present some kind of issue. I thought I would setup a subdomain then build it, then do a 301 redirect on the original domain, but I was told I would lose ranking and links. Then I was going to do the same and just go into the tables and change the domain to the original one but since I haven't done this before I don't know how complicated that would be and I was told it is a pain in the ass. Then I thought I would load wordpress and change the index file to wp-index and then change it back when I was ready. But I don't know if that would work.

Is there some obvious thing I am not thinking of that I can do?