As I dont know what OS you are using on your server, it is
a little difficult to answer this question. I will therefore
give an example for UNIX / LINUX systems...

  You can create something called a symbolic link, this is
basically a server side redirect, achived by creating a
symbolic file that points at a real file. You create your
symbolic link in by using the following command...

ln -s [index.shtml] [index.html]

  I can happily guarentee this technique works perfectly,
because I have used it myself, to anyone going to your old
index.html file it would just appear that you have re-coded
that file...

  If your using another OS you should be able to achive the
same results, I will help if I can, but obviously I dont
know all of them inside out, LINUX is the main system I use.

Hope this helped

Regards
      Brainwipe