Why would an outdoor light fixture suddenly give off dim light even when the light bulb has been changed?

It could be that you have a bad connection behind the fixture. If the wire nut is loose you will have voltage, but not as much current making the light dim. If the connection is bad, then it gets hot, the hotter it gets, the worse the connection can get. You get the picture. Also, the porcelain or bakelite plastic socket the bulb goes into has rivets at the base of it up inside the fixture. Sometimes these can become loose causing the same situation as above.