When creating serializable classes remember ALWAYS to set 
SerializationUID = 1

This makes sure that project are compatible accross versions.

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How to repair a project with broken serialVersonUID?

This is a situation that hopefully only will happen to those using the cvs 
(HEAD) version during development. 

If you have saved a project and when you saved it some serializable project 
class did not have serialVersionUID = 1L, you will not be able to reload it, 
when the serialVersionUID finally is set to 1L. This happened to me now with 
MidiPlayOptions.class...

What you can do - to repair your project is this:

I looked at the exception, which tells the serialVersionUID of the saved 
object, and inserted it temporarily into MidiPlayOptions.

public class MidiPlayOptions implements java.io.Serializable {

        private static final long serialVersionUID = 2770438632652869105L;

Then I was able to open the project - but my goal is to make this able to open 
when the serialVersionUID is what it should be (1L) - so I have to save the 
project with the correct class. You cannot have this two different 
serialVersionUIDs live simultaneously - so you'll have to remove your 
projects references to the "damaged" object.

You can do this by modifying writeObject - so that when you save the project - 
the reference is removed:

        private void writeObject(ObjectOutputStream out) throws IOException {
                this.playOptions = null;

Your project should now be fine to reload correctly. Revert to the correct 
code - and all should be fine...

(AND do NOT commit your damaged serialVersionUID to cvs - it should ALWAYS be 
1L)..