I'm no "expert" but I'm fairly certain that you are incorrect. As I understand it, most - if not all manufacturers - will not void the warranty just because you upgraded the RAM or even installed a new hard drive. I think what happens is that, naturally, they won't cover the new parts under the laptop's warranty. I'm pretty sure that it does not completely void the warranty, in the same way that putting new tires on your car does not void your whole warranty.
If anything, if you do have a problem, they might ask you to return it in its "original configuration" - AKA, with the parts it came with.
I have some Dell laptops here (a few years old) and even the hard drives slide out by removing ONE SCREW, and can be VERY easily interchanged or replaced. I'm pretty certain this was done for the consumer should they feel like upgrading it. The memory slot was also imprinted with a little RAM chip symbol.