Open the presentation you want to move to the new template
Copy the slides you want to move from the old presentation
In the new template presentation, click in the slide navigation area (in the left)
Drop down the paste menu by clicking the small down arrow below the “Paste” button, then select the “Use Destination Theme” icon.
The slides will be pasted in the new template format. There may be some items that need to be “tweaked” but the majority of the slides should move to the new styles.