If you need a new URL, then use that new URL in the links in your pages.
It is links that 'define' URLs.
If the internal filename on the server is not changing, theh use a rewrite as that will connect the URL request used out on the web, with the actual filename used inside the server, without revealing what that filename actually is.
If you have a new URL to replace an old URL, then use a redirect to force requests for the old URL to make a new request for the new URL. However, you still need to use the new URL in the links in your pages, and no longer link to the old URLs.
That was pretty close. I'd suggest: RewriteRule ^(([^/]+/)*)airagency([^A-Za-z].*)?$ http://www.example.com/$1airspace$3 [R=301,L]
The "bounding" subpatterns are meant to prevent this redirect from being invoked if anything but a slash precedes "airagency" or if any letter(s) directly follow "airagency" -- for example, "blah/newairagency/blah" will not be redirected.