Moving or redirecting new site (subdomain) to old site (main domain)

I am looking at doing a major update to both WP and a site’s content. Example:

• domain.com (old, outdated, needs to be replaced)
• test.domain.com (new, needs to eventually be moved replacing the outdated main site)

Is there a way to update WP and a newer version of a site on a subdomain (as a testing site etc) and then move it replacing old site – or redirect it to the main domain?

Are there any things to be aware of when doing this? (or does anyone have any other ideas to update both a site and WP ?)