replacing doublequotes in csv

I've got nearly the following problem and didn't find the solution. This could be my CSV file structure:

1223;"B630521 ("L" fixed bracket)";"2" width";"length: 5"";2;alternate A
1224;"B630522 ("L" fixed bracket)";"3" width";"length: 6"";2;alternate B

As you can see there are some " written for inch and "L" in the enclosing ".

Now I'm looking for a UNIX shell script to replace the " (inch) and "L" double quotes with 2 single quotes, like the following example:

sed "s/$OLD/$NEW/g" $QFILE > $TFILE && mv $TFILE $QFILE

Can anyone help me?