If and only if you have a custom design upgrade and you have CSS edited the theme then CSS is theme specific. If you have custom CSS and later change themes, you need to delete whatever CSS you have in the Editor, otherwise, you’ll get undesired results.
You cannot "kill" any theme you have previously used. I chnage themes so many time every day answering questions that I could never keep count. It takes minutes ot do this > Appearnce > Themes
select theme > preview > activate if you like it
No data is ever lost. You may have to locate some widgets here > Appearance > Widgets > Inactive widgets box and replace them as desired in sidebar and footer areas. You may have to create a bew header image of the correct size for the theme you use.
If you do not choose a theme that has a drastically smaller maximum displayed image width in posts and pages or a dramatic change to sidebar width, there will be no resizing to do and this is all over in a few minutes time.
To ‘reset’ a theme, you need to delete the CSS in the Editor, make sure the “add to existing CSS” radio button is checked, then save. Things should go back to normal.