Nothing is likely to go wrong. If there isn't a serialVersionId defined in your class, the JVM will derive one based on the features of your class. You will have an issue if you serialize an object of a type to some persistant store, change a feature of the class and reload it from that store. The features of the class will be different, so the derived serialVersionId will be different. .
If the JVM that serialized the objects is the same one that will de-serialize them - as is the case with session persistence across app server restarts - then you will probably never see an issue.
If you are serializing your objects and passing them to another JVM - such as an applet or another servlet container - and that JVM has a copy of the class file that was compiled with a different version of javac, you could see some serialization exceptions.