You said you have serialized data. I can imagine that if you have old serialized objects (from when you were using an older JDK 6 version), you might still get that error even when you try to load them when using a newer JDK 6 version.
Are you storing those serialized objects for a longer time? Note that serialization is not very well suited for long-term storage. The problem is that serialized objects have a very direct mapping to the
Java code, so any change in a Java class might make it impossible to read the serialized object back. It's not only a problem with your own source code; if Sun for example would change something in a Java update, it could happen that you suddenly can't read your serialized objects anymore.
Serialization is good for sending objects over a network or for temporary storage (a cache, for example), but not for long-term storage.
You should use a well-defined data format for that which is not tied so tightly to the source code.