Changing domain name

I have a Question just to do it right.

I have a website in WordPress: https://testdomain.com/

This website is in the root folder.

The boss wants his old website name to point to the new website:

https://oldwebsite.com/

What I think I have to do:

1- Change domain pointers of the old website to the new website. EG: ns.testdomain.com

2- Register the old domain into the CPANEL as a park domain.

3- Go to the back end of the New wordpress website, then go to settings, then changed the domain.

WordPress address (URL) old domain instead of the new test domain
Site address (URL) old domain instead of the new test domain

4- Make sure that all images appear. Many of the images have full path of images, and I need to change that, but I do not know how yet. So to preserve images the path now is: https://testdomain.com/wp-content/uploads/2011/01/image.gif

We did try to only do relative links, but some times it did not work.

This website is about 5000 postings. Is huge. I need to do it right.

Please help me out.