OG subscribe link rewrite
Well you know that all these requests are made using hook_menu. 

Yes, you should use hook_menu_alter Create new module and add hook_menu_alter

function my_module_menu_alter(&amp;$items) {
    // Unset core path
    unset($items['The_actual_menu_request']);
    $items['your_new_menu_request'] = array(
        'title' =&gt; 'OG subscribe',
        ...
      );
}


Just pass the same page callback and arguments as the menu request for og subscribe

EDIT

Well try this snippet and see if it works, for OG subscribe menu request

Create a custom module. If you do not know how to create a custom module read This

then in your .module file add this code. It should work, I do not know if the 'file' argument needs an absolute path to the file, but if it does not work you just have to change it for the real path in og '../og/og_ui/og_ui.pages.inc'

function my_module_menu_alter(&amp;$items){
  //unset core path
  unset($items['group/%/%/subscribe']);
  //as an example (taking the example url you provided) I will call the new menu request community/node/63/sign-in
  $items['community/%/%/sign-in'] = array(
    'type' =&gt; MENU_CALLBACK,
    'file' =&gt; 'og_ui.pages.inc',
    'page callback' =&gt; 'og_ui_subscribe',
    'page arguments' =&gt; array(1, 2),
    'access callback' =&gt; 'user_is_logged_in',
    'title' =&gt; 'Join group'
  );
}


Greetings hope it helps you, do not forget to mark the answer.