How to create a WordPress essay environment?


I have an outdated site of more than 500 pages coded manually, which I would like to change to a CMS system: WordPress will be.

But, I would like to keep the old site active for a while.

How could I build the new version of the site in parallel without replacing the previous one?

Should I install WordPress in a subdomain? Can I then (later) move it EASILY to the main domain?

Or, is there another way to test (100%) the WordPress version of the new site?