I understand the reasons why (they added a rich text editor to the "Description" field, but Google has quietly turned off the switch that lets you use the old web UI.
They also, simply force-updated any open sessions. So if you were on the old UI, esp. if you had told Google Calendars you didn't want to switch, well, you woke up (or possibly after making a change), to the new interface.
As a user, I think that's pretty lame, esp. because there are places where there is a learning curve, even if they are improved, they are different.
The part I didn't like was that the new UI has missing functionality. The two areas it broke for me were:
- Color selector in the quick view. We color code our daughter's calendar events. About 75% don't use the default color
- Alarm display in Agenda (or anywhere else). I like to turn off any necessary alarms, and the Agenda lets me skim the list