I had this issue the other day.
OPTION 1: If the template is already tied to the pages in question, all of those pages are now in Approved with Draft mode. Simply go to each page and then Preview and Approve.
OPTION 2: If you need to SWITCH templates, you can't really do that. You must CLONE the LP, then change the template on the creation dialog screen. If the templates aren't too wildly different, this should work well. You'll need to test and edit these new versions of course. You'll then have to setup a 301 redirect from the Original Page to the New Page using URL Tools on the original page.