Your player can not play HD-DVD's anyway there is little chance Toshiba or any of their competion will make software to allow your player to play HD-DVD's because then they would not be able to sell their new $400-$500 HD players.
I have a Toshiba SD-7990 DVD Player hooked to a Denon receiver capable of all the cool surround sound formats (6.1, 7.1, DTS-ES and others) via a digital optical cable. If I buy HD-DVD software, will it play back all those cool formats even though my DVD player is not HD? Will my Toshiba SD-7990 DVD Player even play HD-DVD?s? If it does play HD-DVD?s will it ?down convert? to a 5.1 sound format?