Shell Programming and Scripting

Hi,
I am writing a shell script, which should comment a if-fi loop in a file.
From google i found that, we can use below qualifier to comment a section of code,
:<<COMMENT

COMMENT
But i could not place these keywords before and after if-fi loop.
Any help would be appreciated.
Finally the file should be having like this

:<<COMMENT
if [ "$asdf" = "3" ]; then
----
----
fi
COMMENT

Code:

bash-3.00$ sed -e 's|^if|:<<COMMENT &|' -e 's|^fi|& COMMENT|' test
:<<COMMENT if [ "$asdf" = "3" ]; then
----
----
fi COMMENT
bash-3.00$ cat test
if [ "$asdf" = "3" ]; then
----
----
fi

This is replacing all if-fi loops, so i changed the command to,
sed -e 's|^if \[ "$asdf"|:<<COMMENT &|' -e 's|^fi|& COMMENT|'

this is modifying all "fi" statements.

How to change only that particular if-fi loop?