if you want to switch templates, it is always a good idea to compare the available module positions first.
in your original template you might have module positions that do not exist in the new template, so after the switch some modules may not appear. that means you will have to assign those modules to other module positions that exist in the new template.
so this might be something to consider, but in the end it's not a big deal.
for the technical side of it, just install the new template and assign it as the default template...
> extensions > template manager > select your new template and click "default" icon on the top right of the screen.