- Will I be doing any harm since the site is already indexed?
unlikely, but just incase set index.asp as an alias for index.asp, this way links to your old home page don't break.(I don't know how to do this for IIS, I'm sure someone else will)
- does .asp or .html for the index page have any effect with the search engines?
minimal, but html will load faster than a .asp page, so that's a benefit.