I found this nice handler on a website, and I have often found it helpful. (I wish I could take credit.)
on changeCaseOfText(theText, theCaseToSwitchTo)
if theCaseToSwitchTo contains "lower" then
set theComparisonCharacters to "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
set theSourceCharacters to "abcdefghijklmnopqrstuvwxyz"
else if theCaseToSwitchTo contains "upper" then
set theComparisonCharacters to "abcdefghijklmnopqrstuvwxyz"
set theSourceCharacters to "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
else
return theText
end if
set theAlteredText to ""
repeat with aCharacter in theText
set theOffset to offset of aCharacter in theComparisonCharacters
if theOffset is not 0 then
set theAlteredText to (theAlteredText & character theOffset of theSourceCharacters) as string
else
set theAlteredText to (theAlteredText & aCharacter) as string
end if
end repeat
return theAlteredText
end changeCaseOfText