  The statement of Lemma 3.1 in the published paper is not correct. Lemma 3.1
is needed for the proof of Theorem 3.2. Theorem 3.2 as originally stated is
true but its "proof" is not correct. Here we change the statements and proofs
of Lemma 3.1 and Theorem 3.2. We also prove a new result. Let $k$ be a field,
$A$ a left and right noetherian $\mathbb{N}$-graded $k$-algebra such that ${\rm
dim}_k(A_n)< \infty$ for all $n$, and $J$ a graded two-sided ideal of $A$. If
the non-commutative scheme ${\sf Proj}_{nc}(A)$ is isomorphic to a projective
scheme $X$, then there is a closed subscheme $Z \subseteq X$ such that ${\sf
Proj}_{nc}(A/J)$ is isomorphic to $Z$. This result is a geometric translation
of what we actually prove: if the category ${\sf QGr}(A)$ is equivalent to
${\sf Qcoh}(X)$, then ${\sf QGr}(A/J)$ is equivalent to ${\rm Qcoh}(Z)$ for
some closed subscheme $Z \subseteq X$.

Rings and Algebras