The "Change Slide Template" option has been removed.
The option was removed because many users would lose the content (such as, text, images, video, audio) within their slide during the conversion. Unfortunately this loss couldn't be undone or reverted.
This loss of content was due to a difference in slide options, as most templates have different options and layout.
We recommend creating a new slide using the template you want the slide to use.
Once the slide has been created, copy and paste the text content over to the new slide, and add the desired image, video, or audio elements.