visibility + unserialization

I have a 4 year old framework that serializes objects and stores them in the database. It is now going PHP 5-only, so I began to add visibility to the class definitions of the serialized objects. However, then things didn't work properly after objects stored [before the visibility was added] were unserialized. For example, each protected field ended up creating two fields, with one followed by ":protected". Expected Result: ---------------------- object(Foo) { ["bar"]=> string(0) "banana" } Actual Result: ---------------------- object(Foo) { ["bar:protected"]=> string(0) "" ["bar"]=> string(6) "banana" } Anybody run into this, or can explain why this might be and how I can avoid it? Thanks, ~Ted