Hey I consulted this iFixit guide, the official guide on the apple website and also the crucial memory selector tool.
1) Apple said you didn't have to disconnect the power. So you don't (if you're just swapping out the ram).
2) Crucial confirms that 16 gb works. I ended up getting 16 gb of PNY at BestBuy. They didn't have any Crucial on the shelves.
3) The first two times I did this, the connections weren't perfect. So I had the triple beep motherboard error and then the "nothing happens when i try to power on" error. So if either of these happens, fret not. Just calmly open everything back up and do it again.
So happy I did this. Yosemite was a memory guzzling beast. Now I have garageband and iTunes and iPhoto all blazing and not a hitch.
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