Migration from one url to another url

First off let me state that I am very new using wordpress. I have however been maintaining sites using dreamweaver, etc for many years.

Here is my question.

I currently have a large website up and running it is a .com site

I am currently building a wordpress site to replace it. I am using our .org site for this “test” site. Once I have inputed all the content, formatted, tested it etc. I want to convert the .org test site to the .com url.

I want to make sure all linkage (i am using permalinks) gets converted from .org to .com when I make the move, etc. I cannot really afford any downtime other than the time it takes to take down the .com site….migrate the new data over to that server…and then upload.

Any help is appreciated.