A quick and easy way might be to use the URL Redirect. Open you siteurls.config and add a new XML node with the following:
LOOKFOR
http://www.sampleabc.com
REPLACEWITH
http://www.samplexyz.com
However, you should know that this will change it across the board, so your original or parent site would no longer be accessible unless you commented out or removed that rule.
I would suspect that you need a more flexible solution. What I suggest is determining what your business rules are and creating a module around that. For instance, if at IP # xxx.xxx.xxx.xxx, show the parent portal, otherwise, redirect all users to the child portal.
So you would build the module with that logic, and then install it on your parent portal. When it is installed, place it on the home page of the parent portal, then using the module settings, display it on all pages.
[EDIT]
I just realized that there is an even easier way to do the same thing. Just put the same logic into your skin file instead of a module. That way, you do not need to create a module package (if that is too much for you).
[/EDIT]