Moving WP installation from subdomain

Hi there
I am going to use WP on an existing site, and I am wondering if I can install WP in a subdomain > test & evaluate > and then later move all files to the main domain.

Reason: I want to work on WP while the old site (not WP) is still running.

What’s the catch? wp_config? Permalinks? Many issues? Impossible?
A better way of testing the new WP without stopping the old site?
(I know WP pretty well, but no experience with this issue)

Thanx
Jesper