Update destroys website-layout

I spent many hours customizing your theme to my customers needs. I created and activated a child theme. Fine. Then came your update, and it completely destroys the design. Why couldn’t you just make a new theme and leave the “old” one alone? After the update, the theme doesn’t even recognize the correctly integrated Google fonts, let alone the page-templates I created.
Is there any other solution than going back to the old version of the theme?
Kine regards