Well done! I figured out how to go straight to the bulk edit page so the user doesn't have to find it in the context menu. It requires the search page to be requested first, then from there the link to the bulk edit page will work. I have created a gist for it, so it can be updated without having to post new comments here. See https://gist.github.com/4552951. Again, it has only been tested on 5.2.1.

Just found out about an easier way, which should work in all browsers supported by JIRA.

Just paste this into your JIRA Announcement banner:

<script type="text/javascript">

AJS.$(document).ready(function(){

    var matches = location.pathname.match(/browse\/([A-Z]+-[0-9]+)/),
        list, id, item, link;

	    if (matches){
			id = matches[1];

			list = document.getElementById('subtask-view-options');
			if (list) {

				link = document.createElement('a');
					link.href = '/issues/?jql=parent%20%3D%20' + id,
					link.className = 'aui-list-checked aui-list-item-link',
					link.title = 'Show and edit subtasks in navigator',
					link.innerHTML = 'Edit All';

				item = document.createElement('li');
				item.className = 'aui-list-item';
					
				list.appendChild(item);
				item.appendChild(link);
			}
		}
	})

</script>

Yes, thank you. I forgot to mention that I made it for my installation, which is 5.2.1, and can not speak for what other versions it works on, as I don't have a way of testing them.

Thank you for the user script. It doesn't work as expected however in 5.1.5. Is is it supposed to work in 5.1.5?

When I use it on issue ILUK-1828, it generated a link to https://jira.isaac.nl/issues/?jql=parent%20%3D%20ILUK-1828.
I think this is the 5.2 style navigator?

I've attached a userscript which works for 5.1.5 (and possible older versions as well)

Also, you may need to modify the regular expression to suit the naming convention of your projects.

I have added jira.search-subtasks.user.js as an attachment to this ticket. It's a user script to open all subtasks in search, where Bulk Change is available to them. You will need to change @match to reflect your own installation of JIRA. I have only tested it with my own JIRA installation, and can not speak for which versions it will work for. It works differently from Joel's script – it adds a link from an issue's page to a search containing all the subtasks, instead of selecting existing items on the search page.

I have attached my greasemonkey userscript.

When you install it you will need to update the include line for your jira installation.

Currently it is:
// @include http://your.jira.installation/secure/views/bulkedit/BulkEdit1*

But if you wanted to use it on this jira installation you would change it to:

// @include http://jira.atlassian.com/secure/views/bulkedit/BulkEdit1*

One thing worth noting, is that it can only select subtasks on the same bulk edit screen. If you are on the bulk change page and some of the sub tasks are not present on the page then they won't be selected.
So you need to make sure that your search query will include the sub tasks and their parent if you trying to move/modifying a task with all it's sub-tasks.

I am interested in the script for Firefox. Please attach it to this issue.

If anyone is interested I have hacked up some javascript for greasemonkey in Firefox, which adds links on the bulk edit page next to a sub task that says "Select other subtasks".

Then if you click that link it selects any other sub-tasks on the page including the parent task.

A "bilk-change sub-tasks" action would be great: not only one can transition them but it also happens that one needs to reassign them to a different person or schedule for a different version.

In all these cases one has to update the sub-tasks one-by-one