Does Upgrade from 2.20 to 2.40 break themes / custom views?

I have some heavily customized views and themes. Will upgrading break them?

Really, nobody knows that answer to this question?

It would depend on your customizations really.
The best way to find out is to build and run your themes on a 2.40 installation. You might get some compilation errors because there were some changes to the api of the models and such. But generally they are straightforward to fix. If your theme is done as a proper nopCommerce theme it should be easy to upgrade.