How to re-evaluate cl-defstruct during same Emacs session

Question

I am putting together a modification of a library that contains an existing cl-defstruct definition, and I am having problems redefining it after its initial evaluation.

Q:  How can I replace the old definition with the new one? [It would be nice if I could wrap it in something so that eval-buffer, eval-region, and/or require will be sufficient to force its re-definition.]

BEFORE:

(cl-defstruct
  (undo-tree-node
   (:type vector)   ; create unnamed struct
   (:constructor nil)
   (:constructor undo-tree-make-node
                 (previous undo
      &optional redo
                  &aux
                  (timestamp (current-time))
                  (branch 0)))
   (:constructor undo-tree-make-node-backwards
                 (next-node undo
      &optional redo
                  &aux
                  (next (list next-node))
                  (timestamp (current-time))
                  (branch 0)))
   (:copier nil))
  previous next undo redo timestamp branch meta-data)

AFTER:  Adding the word count at the very end.

(cl-defstruct
  (undo-tree-node
   (:type vector)   ; create unnamed struct
   (:constructor nil)
   (:constructor undo-tree-make-node
                 (previous undo
      &optional redo
                  &aux
                  (timestamp (current-time))
                  (branch 0)))
   (:constructor undo-tree-make-node-backwards
                 (next-node undo
      &optional redo
                  &aux
                  (next (list next-node))
                  (timestamp (current-time))
                  (branch 0)))
   (:copier nil))
  previous next undo redo timestamp branch meta-data count)