First of all, Download your backup before you go ahead with any major change such as changing the theme.
So, whenever you change a theme or switch to a new or another theme, the basic aspects of your store such as
- Product Detail page content
- Write-ups you made in the store
remains as it is! The above aspects do not vary as you switch themes.
Basically, all the images you set up on your store be it slider images, banner images, and all those images that you had set up using Online Store > Customize will not be visible once the switching is done but you won't lose the images. You need to go to Online Store > Customize and Select Images and assign all the images that should already be saved in your store as you had used them in your previous theme.
And if you find any query in general, please connect with me on firstname.lastname@example.org