How to replace an existing 'tab' /module

@poncho, I think you are complicating yourself for a simple thing.  You wanted to replace the Tabs for example Purchase with your custom module of Purchase, right,   

Anyhow you have a hook.php file which will inherit the application and create new constructor to create tab for your custom modules. Just use the exiting Slug,it will replace the existing with your module.

Here is my example code for you to understand it .

 
class Kvcodes_app extends application{
    var $apps;
    function __construct()  {
      parent::__construct("AP", _($this->help_context = "&Kvcodes"));
    }
}

This will simply replace your purchase module with Kvcodes and here you have to add the further functions. to replace the links under the menu,

@kvvaradha, that works perfectly, thank you. Although I'm not 100% sure how it does though!
Edit after a little thought -
So it is declared using the new ext 'class Kvcodes_app extends application'
then construct the new extension but label it AP -'parent::__construct("AP"...
Then as @notrinos states although the module AP exists calling it again redirects to the new ext?
Is that right?

Now I know I was close last week...I think I tried redeclaring .... 'class AP_app extends applica...' which (I now realise) obviously will not work.

But part of me still thinks apmuthu had something else in mind when he wrote:-

Which to me means that the security code has to be re-defined...doesn't it?

@poncho, You dont need to get confused much with this.  Because the extension hook will replace the existing functionality.  The Perfect example is Access security roles. When you define a custom one but it uses the existing system access role numbers, it will override the access permission with new one.

Last, Yes the Wiki talks about the security roles and access permissions.