See Here for the full version of "For The Fallen" the most common misquote of this (as repeated by the Walrus) is "They shall not grow old" which should read "They shall grow not old"
Unfortunately, this misquote is used in several of the websites which quote the poem in full, so perpetuating the difference.
I always notice this, because of its appearance in the responses for the Remembrance Sunday service used by the Anglican Church. Whenever someone re-types the service, or attempts to up-date it, they (almost) invariably 'correct' the quote!