The difference operator is the endomorphism $\delta$ of $\mathbb{R}_n[X]$ such that $\delta = \tau - \operatorname{Id}_{\mathbb{R}_n[X]}$:
$$\delta : \left\{ \begin{array}{l} \mathbb{R}_n[X] \rightarrow \mathbb{R}_n[X] \\ P(X) \mapsto P(X+1) - P(X) \end{array} \right.$$
For a non-constant polynomial $P \in \mathbb{R}_n[X]$, express $\operatorname{deg}(\delta(P))$ and $\operatorname{cd}(\delta(P))$ in terms of $\operatorname{deg}(P)$ and $\operatorname{cd}(P)$.