It's not very hard if you've been inside a computer before but there are many wires you have to handle carefully to avoid issues.
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Keep in mind that opening the case voids warranty
There is some question about opening the case and voiding the warranty. It's generally accepted that it won't void your warranty unless you break something, or your modifications break something. (e.g. overclocking)
Your Mac mini doesnt have any user-serviceable parts. Do not attempt to open your Mac mini. If your Mac mini needs service, consult the service and support information that came with your computer for instructions about how to contact an Apple Authorized Service Provider or Apple for service.
If you open your Mac mini or install items, you risk damaging your equipment, and such damage isnt covered by the limited warranty on your Mac mini.
There is some question about opening the case and voiding the warranty. It's generally accepted that it won't void your warranty unless you break something, or your modifications break something. (e.g. overclocking)
I used a pizza cutter to open up mine. No square or sharp corners that could put scratches on the case while opening it up. Just be careful with the airport and bluetooth antennas.
It tells you to avoid attempting to opening it up but it doesn't say the act will void your warranty.
If you open your Mac mini or install items, you risk damaging your equipment, and such damage isnt covered by the limited warranty on your Mac mini.
Now if you do open it up and damage something then your warranty could be VOID. It's all up to the service tech to decide that. I don't ever recall seeing one post anywhere with the subject "I opened my Mac Mini and voided my warranty".
From http://manuals.info.apple.com/en_US/Mac_mini_Late2009_UG.pdf
That pretty clearly says that it'll void it, or at least give Apple an excuse to refuse to fix it
It does not say it will void it. It says if you do any damage, that damage is not covered.
If Appld can't tell that you opened it, they cannot void the warranty. I have never been denied service on any of many Minis. All of which have been upgraded in some way.
And it is not hard to prove that opening it didn't break anything. In fact, the burden of proof is on Apple, not the consumer. If Apple can't prove that it was damaged while being upgraded then they can not void the warranty.
A little off the original topic, but I was wondering if Apple does decide to not cover something for this very reason, do they actually make a note somewhere that will blacklist your serial number?
If not couldn't you just try say another Apple store or an authorized service provider until you find somebody who doesn't care?