1) The Spacebed is very comfortable unless you are used to traveling in First. I've slept almost all the way from EWR to SIN in it, waking up only for the meals. From my experience, I only experience slippage on the Spacebed when wearing business trousers. When I wear jeans or the SQ PJs, which is most of the time, I have never experienced slippage.

2) Yes, for a fee in $ or miles to redeposit the mileage. Check the SQ website. Your economy class booking is canceled when you upgrade, so you may not have an economy class seat available anymore on that flight when you cancel the upgrade.

3) You can change the route if your economy class ticket allows you to. Obviously, any upgrade you have on the nonstop SIN-LAX will be canceled and will have to be redeposited with the appropriate fee, and you will need upgrade availability on the SIN-NRT-LAX flight to upgrade.