toggling between permalinks settings

When establishing the “permalinks” settings …

i. If I select the “Default” setting option all of my existing posts [100’s] and pages are displaying properly.

ii. If I try and switch to the “Day & Name” setting option none of my existing posts and pages are displaying at all [i.e. error message reads as Page Not Found].

Question 1. Any idea what the reason for this is?

Question 2. How do I fix the problem so that I can use the “Day & Name” setting option permanently?

TIA