Upgrade from 4.1 -> 4.2, how to handle theme

Hi

Installed a 4.1 site last year and was thinking about upgrading to 4.2 to see if I can get an performance boost.
The theme I used as a buildingstone was "bootstrap default clean" (https://www.nopcommerce.com/default-clean-bootstrap-version) but have altered it to match a custom bootstrap theme for another site.

Is there any way to know what I should upgrade in the template if I upgrade to 4.2, or do I need to take a 4.1 default theme and do a compare to a 4.2 default? Also, will anything break?

Compare  4.2 default theme and your 4.1 theme and look the changes. I think you get the better idea about changes