I don't know how you're accessing tag but the following works for me:
import BeautifulSoup
if __name__ == "__main__":
data = """
<html>
<h2 class='someclass'>some title</h2>
<ul>
<li>Lorem ipsum dolor sit amet, consectetuer adipiscing elit.</li>
<li>Aliquam tincidunt mauris eu risus.</li>
<li>Vestibulum auctor dapibus neque.</li>
</ul>
</html>
"""
soup = BeautifulSoup.BeautifulSoup(data)
h2 = soup.find('h2')
h2.name = 'h1'
print soup
Output of print soup command is:
<html>
<h1 class='someclass'>some title</h1>
<ul>
<li>Lorem ipsum dolor sit amet, consectetuer adipiscing elit.</li>
<li>Aliquam tincidunt mauris eu risus.</li>
<li>Vestibulum auctor dapibus neque.</li>
</ul>
</html>
As you can see, h2 became h1. And nothing else in the document changed. I am using Python 2.6 and BeautifulSoup 3.2.0.
If you have more than one h2 and you want to change them all, you could simple do:
soup = BeautifulSoup.BeautifulSoup(your_data)
while True:
h2 = soup.find('h2')
if not h2:
break
h2.name = 'h1'
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