Without deleting the old one, is there any way to manage through host headers/ alternate mappings?
So could you clarify - you want both web applications to use the same host header? This is not possible but if you want the new web application to use www.abc.com and the old one to respond to a different host header (www.oldabc.com) this can be achieved.
Is this what you are after?
The basic steps are:
You will need to re-run your search crawls and a few other things but that is the general process to follow. The main issue is going to be if you have lots of site collections and lots of content databases then you will need to do steps 3. and 5. for all of them.
Is the current www.abc.com website based on Sharepoint? If not, the easiest way is to create a different Sharepoint/web farm on which you install Sharepoint. That way the current webserver can still host the www.abc.com and the new Sharepoint farm can create a site with the hostheader www.abc.com also. As long as your internal DNS or hosts file(s) don't mess things up you'll be able to do this.
If the current site is based on Sharepoint, maybe the easiest solution is to create the site on a different port, so www.abc.com:81. I'm not much of an ISA/Forefront expert, but I think you can set those up to redirect a request to the proper site:port.
Other solution might be to just 'Stop' the current website in IIS. That way a new website will be created with the hostheader. Just remember to backup your current virtual directory and other stuff you've configured for the current website.
Also Nick Hadlee his suggestion is a good solution to your problem.
This was the step i used
Created a new Webpplication www.newabc.com and in the alternate access mapping for this i used www.abc.com. and i stopped the www.abc.com which was already there
One thing i have noticed is that when we add the alternate access mapping . It did not update the host entry in IIS. I had to manually do this after that this worked