Redo site - effet on Google search results - links change

I am moving our site to a new server and in the process completely redoing our site. What if some of the links change?

Grossly simplified Example:
about.html

may change to

aboutus.html

How will this effect the google search results? Our business is on the top 1-2 pages for several common searches related to our business…and I don’t want to lose this. So if someone clicks on a search result, will it say "page not found" or will it default back to our main page?

One step further…

Our business name is also changing, so we are going to use a new domain.

Back to the example…

Current page would be

current.com/about.html

And this will change to

newname.com/aboutus.html

What to do…

If I redirect the old domain to the new domain, will it just redirect any traffic to ANY page on the old domain to the home page of the new domain?

***I still control both domain names, even though the name changed. Maybe I just leave the old site as is and just plaster links to the new site all over it?
Just realized I have fat fingers…obviously meant to say EFFECT on google search in the title.
.htaccess 301 redirects on the old domain to the new domain and file locations. That will preserve your google ranking

something like this in the .htaccess file on your old domain

redirect 301 /about.html http://www.newdomain.com/aboutus.html

for each page

Thanks!

Is is possible to set something like this up for all the pages at once?

Or…a redirect for ANY link that doesn’t exist anymore?

I guess you could just set your "page not found" to be your home page, right?

Yeah.. I don’t know the code off the top of my head, but if you Google "404 .htaccess redirect", it’ll give it to you.

It’d be more beneficial to do about.html to aboutus.html and contact.html to contactus.html so that contact.html isn’t redirecting to index.html, which will make Google be like "the fuck".