I have a site that I recently updated the main template on and it didn't update all of the sites.
All of the sites? Do you mean all of the pages? Sometimes when a changed template doesn't update all the files you expect, you need to prompt Dreamweaver to reapply the template - in my version of DW it's Modify/Templates/Update Pages
When I was re-editing my sites some of the pages needed to be cleaned up so I detached them from the template, cleaned them, and then uploaded them (the template looks like it's still there!).
Yes, it will look like the template is still there, but the file no longer contains template directives, template content and your page content, it only contains template content and your page markup. In a browser it will look identical, as the directives are styled as HTML comments. But it isn't identical - it has no directives.
Did this detach all the pages from the template updates?
It will detach all the pages you told it to.
Is there any way to reattach the templates without copying all of the editable region and then pasting it into a new template shell one by one?
No - the information about what goes where is lost - you removed it. There *are* ways to reattach a template, but only if you roll up your sleeves and type the directives in by hand (or copy them from a page that still has the template attached).
As it is there is modify/attach but that just puts a new template around the old one.
Yes - if you have a file with template (a) attached, you can attach template (b) instead - each editable region is moved to the corresponding new place in the new template, and those that have no corresponding new place are offered to you to choose whether to move them to a different region, or to delete them.