[Zope] Adding new _properties

[Serg]

I had product with below properties:

class easyArticle(SimpleItem.SimpleItem):
    _properties = (
        {'id':'author', 'type':'string', 'mode':'w'}
    )
...

I inputted some objects of this product.

Later I added some properties:
class easyArticle(SimpleItem.SimpleItem):
    _properties = (
        {'id':'author', 'type':'string', 'mode':'w'},
        {'id':'n1','type':'float', 'mode':'w'},
        {'id':'n2','type':'float', 'mode':'w'}
    )
...

After that, I cannt read old objects of this class. Zope write:
Site Error
An error was encountered while publishing this resource. 
Error Type: AttributeError
Error Value: n1

How can I solve this problem?

Regard,
Serg.