The HD input on my 9905 wouldn't take a 480p DVD until it was repaired/upgraded. Not sure which, since they changed two modules, and afterward there was no longer a complete sync loss with a 480p DVD, just great images. Could have been a glitch just in my set, two years old this summer. The 9905's DVD component inputs only take 480i, putting a half-doubled image (screen split vertically) on the screen with 480p.
Haven't tried the setup you're aiming for, but it's my impression that with externally switched 1080i-or-480p inputs to the HD component inputs the 9905 would display either, smoothly (providing the menu settings for sync, etc. weren't different between the two.) (I'm using the HD jacks for my HDTV cable converter now and have temporally shifted my DVD component outputs back to DVD component, selecting an interlaced rather than progressive out from my player. A way of side-stepping this, according to the 9905 design engineer, would be to use the VGA input for the 2000HD cable converter and the HD input for progressive DVD again.) -- John