Applescript to convert all your Evernote tags to lowercase (tested on osx)

evernote_tags_to_lowercase.applescript

tell application "Evernote"
	try
		-- Get list of Tags
		set theTags to tags
		repeat with theTag in theTags
			
			set tagName to name of theTag
			try
				set lowerCaseString to do shell script "echo " & tagName & " | tr '[:upper:]' '[:lower:]'"
				set name of tag tagName to lowerCaseString
				log lowerCaseString
			end try
		end repeat
	end try
end tell