The physical change of a drive is will be more involved than changing a drive in a laptop or PC case though because the home cinema kit isn't modular. However, it's not impossible just as long as you can find the exact same model as an off the shelf product. Where you may well run in to trouble though is with the firmware.
It's pretty common to find that drives in home cinema kits run bespoke firmware specific to the particular AV system. I'm not saying it will definitely be the case, but it is something you should be prepared for as a possibility. If that is the case, then I'd say it's something of a dead end. The idea of changing to Blu-ray is similarly thwarted. Even if you can get it to recognise the drive, the system might not have the processing circuitry to deal with a raw 1080p signal and it certainly won't have Dolby True HD and DTS-MA decoders for the sound.
A better solution might be to use a Blu-ray player and connect to the HC kit with an optical cable. If you have a PS3 then you can do this right now. Set the BD player menus to output audio via Optical in BItstream format. Connect a HDMI direct to the TV. That's you in business.