Lets say that there's an entity like this:

type: object
subtype: blog
class:  ElggBlog

Copy the ElggBlog.php class to your plugin for example as ElggMyblog.php, make modifications to it and then register the blog subtype to use your class with this function:

update_subtype('object', 'blog', 'ElggMyblog');

If the ElggMyblog class is found from the classes/ directory of your plugin, the blog entities now use your class instead of the original ElggBlog class.

This function needs to be run only once so but it to a file activate.php in your plugin. This file is run when the plugin is activated. Also add a file deactivate.php which updates the class back to the original, so that nothing gets broken when you deactivate your own plugin.

You can see an example of the activate.php in the blog plugin.

is it not simpler, easier for you to extend the original class and code your extensions, overides like in proper OOP ? you just are bt allowed to subtract variables, functions from extended class.

Got to say that Dhrup's approach is better. For some reason I didn't think that far.

huhh ? ;-)

I guess that maybe we just 'read' into Dominic's situation description differently..
and I happened to be lazy and not want to read / quote elgg code in order to answer ;-P

and
typo in my post =
'..just are not allowed to subtract.. '

On how to change what class file is used, calling elgg_register_class() with a new location should overwrite the old location.

Hi Guys 

Thanks for your responces,  I did think about using OOP inhertiance but that requires a new class name and I did not want to go and change all the references that instantate old class with the new class but @cash solution has worked really well. 

Once elgg_register_class() called in the start file the new class definition was loaded.

thank you