You shouldn't, what you did was correct, but you need to change the names in the right place in Lexicon Management and then clear the site cache.
The lexicon strings you want are in the Login namespace *and* in the updateprofile topic.
Also, be sure your Tpl chunk still has the fully qualified language tags, like this:
[[!%login.fullname? &namespace=`login` &topic=`updateprofile`]]