I have confirmed this in 1.9 and the 2.0 Alpha Sandbox.
Steps to reproduce:
1. Create an event.
2. Register enough participants to fill the Maximum Number of Participants
3. Go to the Edit Event Registration page (e.g. /civicrm/contact/view/participant?reset=1&action=update&id=76&cid=103&context=search) for any participant
4. Make any changes and try to save.
5. You will be told the event is full in a red error box:
"Please correct the following errors in the form fields below:
- It's Full!" (Where the second line is the Message if Event is Full)
Therefor, if the event is at capacity, it is not possible to change a participant's role, registration date, notes, etc. Ironically, you cannot change the status of a registration to "Cancelled" from this screen because the event is full.