changing the url...do not want a redirect

You are asking about changing the host value, not the URI. The format of a URL is the following

http://[HTTP::host][HTTP::uri]

http://www.foo.com/foo/bar

HTTP::host -> www.foo.com

HTTP::uri -> /foo/bar

So, for your specific question, you'll need to tweak the Host header. For a host modification, you can do the following:

when HTTP_REQUEST {
  if { [HTTP::host] eq "123.abc.com" } {
    HTTP::header replace "Host" 123ext.abc.com"
  }
}

This will modify the Host header of the HTTP request to the backend server to be "123ext.abc.com" so it will effectively look to the backend server like the request was http://123ext.abc.com/....

Hope this helps...

-Joe