Copy the number from a document to create the number for a change object?

We have a process where one document is placed on a change process (sub-type of problem report) and the users would like the document (that they can enter a number on creation) and the process to share the same number.

Other than just allowing the user rename/renumber, do you know of a way to have that document number populate the change object number?

Any ideas would be great.

Thanks.