You can swap drives. You can swap memory. You can swap battery.
ROG G551 upgrade guide here.
Asus tech support will routinely inform you that anything which involves removing screws voids Asus warranty, but don't worry about the few screws you might need to unscrew to access the user-serviceable (drive bay, memory slot, battery) compartments.
You will lose warranty if you disassemble the laptop, remove any screws covered by anti-tamper stickers (like the "Warranty Void If Removed" label, lol) or strip any screws fastened with threadlock.
Many people report improved thermal performance after disassembling their ROG laptops and doing a full re-paste with a premium TIM (which will outperform the cheap Asus factory pink goop), but this is an involved procedure which can easily break something. A lot of people (myself included) will just seize fate by the throat because confidence in Asus warranty is low.
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