You are probably better off not to use a new ClassLoader, but instead to rename the class and then you can track down the old objects and pass them to the new constructor to upgrade them. You then have to update all the references to make them point to the new object. By using one layer of indirection, you can make that task easier since there is only one link to update. You have to ensure old and new share a common base class or interface that you can use for references.
Another possible object-upgrading tool is serialization where you serialise version-marked objects and reconstitute them with new code. It is slow but convenient. I have not experimented with any of these techniques personally.
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