After the upgrade, they're both there. Which one do we keep?

Anyone from staff?

I went with default, but it would be good to get an official explanation.

Personally I would say 4.2 is the new default as default already existed and 4.2 was created by the upgrade.

Simple

If you have previously edited any templates in your theme, then you must upgrade them manually, or reverting back to default, to do a quick search of your edited templates

 

If you haven't edited any templates then you are ok to do nothing or removing the one which automatically created after the upgrade 

 

I went with the Default.

I kept the 4.2 as it was set as admin cp default.