How to limit sed change to first set
<p>I have a text file that contains entries shown below. </p>

<pre>
tag1start
text1
text2
text3
text4
tag1end
..
..
tag1start
text5
text6
text7
text8
tag1end
</pre>

<p>I need to change entries only in the <strong>first</strong> set of tag1start tag1end</p>

<p>I have attempted using "range" but it affects both the sets. How do I limit it only to the first set ?</p>

<pre>
/tag1start/,/tag1end/c\
Some other text1\
Some other text2\
Some other text3\
Some other text4\
Some other text5\
Some other text6\
tag1end
</pre>