When need to apply a template to an existing presentation arises, start fresh with a blank presentation. Select File >> New. Under New, navigate to the personal or workgroup template and search for the template or choose one of the featured or themes from the gallery.
Open the file containing the existing slides, and the thumbnail pan to choose the slides you need. Copy all selected slides and rightclick on the thumbnail pane, situated under Paste Options, and choose Use Destination Theme. All the new presentations will be copied to the new presentation.