Hey Tal,
When you update a template and approve it, any pages already using that template will get unapproved drafts attached to them. The live approved versions of the landing pages will not update to the new template until those drafts are approved. However, I do not reccomend editing a template that is currently in use if you don't want to update those pages to the new template. This can cause confusion in the approval process, and requires any subsequent changes to landing pages using that template to either adopt the new version, or unlink from that template by editing the HTML of those pages individually. I suggest instead, cloning the template that you want to update, and building the new landing pages on top of that template. This will avoid any confusion in the approval process, and save you countless headaches if you want to update the old style landing pages without updating the styling.