Changing your brandmark in any way takes a lot of courage and stategic planning. Especially if it's a well established mark. Sometimes, an update and complete revamp is long overdue (Holiday Inn, Payless, Xerox), sometimes you can makes significant changes, but still retain the essence of the original, highly recognized version (Armor-All, KFC, Reader's Digest, Compaq), and sometimes the changes are barely noticebable, but highly strategized (Toys R Us, Old Navy, Qantas, Delta). In any case, rebranding is a very interesting study. Here are some good ones:
(P.S. I still contend that the NFL Logo still needs a couple minor tweaks)