Hi anthony,
I'm not so sure that you can simply label mythology as fiction and assume that it can simply be changed at will.
The mythological foundations of cultures have a profound effect on the people of those cultures. You cannot simply make up new ones and slip them in without people noticing.
To use some reasonably contemporary examples (and I'm not examining the religious truth of this here Mods, honest!) if one takes the story of Jesus it has been made to fit pre-existing elements of Judaic myth/religious belief. A simple rewrite would have been unacceptable - you couldn't simply have the New Testament and throw old the OT - the two had to marry.
A less controversial exampel is the Arthurian cycle and the Matter of Britain. Each new iteration adds to the core myth but does not remove it. The fundamental story - the young leader that unies the country, defeats the Saxons and then finally falls because of internicine waar, remains.
When you think how riled a certain section of people get (OK,the odder section) gets when writers deviate from the Star Trek or Dr Who mythos you'll see wher I'm coming from.
Pete
God is our guide! from field, from wave, From plough, from anvil, and from loom; We come, our country's rights to save, And speak a tyrant faction's doom: We raise the watch-word liberty; We will, we will,we will be free!