So ...

Basically, you'll have two options: to perform general, site-wide actions, or to regenerate image sizes only for selected images.

Let's see the first option ...

If you go to Tools => Regenerate Thumbnails, you'll see a "Regenerate Thumbnails" button for any image size defined by your previous theme, or by you under Settings => Media. Here are the available options/sizes in my case:

Pressing the regenerate thumbnail button(s) will start generating the new image sizes according to your new settings/theme. And don't forget, it may take a while, depending on how many images you have in your media library ...

OK. Now let's see the second option ...