 must be avoided.  If
the old <a href="ref-glossary.html#gl:class">class</a> of <var>instance</var> and <var>new-class</var> are both unit
classes that use the global instance-name counter, the old instance-name value
can be retained safetly.  If <var>new-class</var> is a unit class with a
class-based counter, specifying a new instance-name value (using
<nobr><b><a href="ref-next-class-instance-number.html">next-class-instance-number</a></b></nobr>) is recommended.
</p>
<p>When the old class and <var>new-cl