You cannot change their effects.
What you can change is the name, its icon, the prereq, and the expiration tech (for wonders).
Maybe that will be allowed in CIV 3.
No; the effect stays the same even if you change the names, icons, et al.
Other changes in the game can affect some of them, though - if your scenario disallows the space race, the Apollo Program just reveals the map; and the Manhattan Project needn't allow you to build nukes, but whatever unit you put in their place.