How to launch new wordpress site on existing domain

Hi,

I’m creating a wordpress site for a friend but have a couple of questions on how to go about it. I have created a number of wordpress sites before, all of them for startups so no existing site of the new domain. My friend does have content on her site currently so will have build up ranking and wants to keep the domain but change the site in both content and style. We’d rather not take the site offline for the duration of site development as I’m doing bits and pieces here and there so it might take a couple of weeks before we can go live.

I have now installed the wordpress theme on a ‘test’ domain I bought some time ago. It allows me to play around with the theme and content. Plan would be to get the site ready on this ‘test’ domain with my own host, than migrate it to her domain & host. Is this the easiest way to go about it or am I overseeing issues / easier ways? And what do I best do with the old content once the new site is ready, thinking SEO & ranking?

Thanks for thinking along ??