Change theme

Hi. We have a WP blog at
https://www.medicaldentaltourism.net/blog/
We’d like to quit using the current theme (or template?) because
it shows all the post instead of just the lead-in and
the invitation to leave a comment is not at the end of the posted article but up near the article header.
We assume there’s at least one theme (template?) that shows only lead-ins (until the visitor clicks on the article) and makes the possibility of commenting more obvious.
Perhaps Nucleare is such a theme.
https://wordpress.com/theme/nucleare/support
Our question is,
for somebody who’s never done this before, is it safe to swap out the old theme and pop in a new one?
Or template, if that’s what’s supposed to be switched?
Thanks.