Stock HU? If yes not bad la. Can last 9 years. Mine mati after 5 years and buttons sometimes goes crazy.

Can. But why would you want to? Unless you have access to it cheap? It might cheaper to go aftermarket.

Why wouldn't it be ok? For all you know after doing the sound deadening your existing system may improve!

The best is you don't upgrade until warranty over. Because these unscrupulous SOBs can blame your engine break down on your ICE or seats misalignment on your speakers.

Edifier makes car audio? What exactly does Edifier have?

Depends on how unscrupulous the dealer wants to be. I've seen some of the dumbest accusations and claims ever. Like changing alarm was the cause of the VW gearbox problem.

Or sound system install caused the Kia Cerato to have an ECU failure (which is likely if the installer was an idiot).

So if you're worried about warranty, don't touch anything. Otherwise just go ahead and do it and don't care about the warranty. But at the very least get it installed at a shop that knows what they're doing.

There's a shop in SS2 PJ called "VPS" or something, the installer was the ex-CV Audio shop previously from Uptown Damansara. He closed the Uptown shop to move to SS2 because of the ridiculous parking and rising cost of doing business there. They specialise in high end cars and all so your car shouldn't be an issue. They're called Petes audio (look them up in FB). They haven't done their signboard yet so just lookout for the VPS signboard.