I have few questions about moving Roland digital piano. I'm going to use my car, and to fit the piano in it I have to turn the backseats down. I also have to rotate the piano so the back of the piano is on the floor of my car. Could this be harmful for a digital piano?

I'm also wondering if this specific model can be disassembled in smaller units for moving. I tried to searth for this but i could'nt find info. I do know that music rest can be taken off tough, but how about the rest of the piano?

I managed to fit a HP-207 into my compact car a few years ago. With the passenger seat folded and slid up all the way forward, I successfully moved the piano damage free (twice actually). Only damage was to the back of the seat - got torn by one of the corners of the piano rubbing against it.

I can't see how placing a piano any specific way could be of harm, so long as it's not being slammed around inside of your car. As for disassembly, you're looking at removing the unit from the stand / speaker box, and breaking the stand down into smaller parts - that's about it.

How is the piano attached on the stand? I thought they are not actual "separate" parts as the piano is a upright home model. Do I need some tools to take it off? How about breaking stand into smaller parts?

The piano isn't here, I'm supposed to pick it up tomorrow.

Download from Roland US the manual of your piano, there is a brief description of the installation process in it. If they don´t have the manual of your model, download the one of the HP 500 series, it should be very similar.
Find the 4 screws (at least on the HP-500) on the bottom of the piano unit and lose the piano from the stand. Then, with the help of a second person(!), take it of the stand sliding it to the back. You then could disassamble the stand to even smaller pieces as needed.
When assambling everything again, take care to not forget to properly readjust the stabilizer under the pedal again, once the piano is at its new loction.
Should be a straight forward process without any special difficulties to expect, and transport shouldn´t do any harm as long as you don´t bump the piano somewhere and well damp it from the shocks of a bumpy street.