Why the suit should never change!
The reason the suit never changed in all those years (400+) was to maintain the illusion that the Phantom is exactly who they say he is.
The Ghost Who Walks...Man Who Cannot Die...
Pirates, criminals, murderers all believed the Phantom was the same person (or ghost) because the suit remained the same.
From Wikipedia:
"Making a costume based on the image of an old jungle idol, he became the Phantom. When he died, his son took over the role of the Phantom, and such the mantle would be passed down to new generations, leaving people to give the mysterious figure nick-names such as "The Man Who Cannot Die", "Guardian of the Eastern Dark" and "The Ghost Who Walks", believing him to be immortal."
http://en.wikipedia.org/wiki/The_Phantom
http://www.toonopedia.com/phantom.htm
Changing the suit destroys the myth.