As easy as upgrading a classic Mac.
I had an '07 iMac and for various reasons (free upgrade, iCloud, iWork, etc.) I upgraded from 10.6 Snow Leopard to 10.9 Mavericks. While they are compatible, 10.9 slowed my machine down quite a bit. I was considering buying a new machine, but I feel like this one still has some life left in it. It still had the stock 250GB HD and given how Mavericks uses the hard drive I figured a 2TB upgrade would help improve the speed. It's a much smaller investment and if it didn't help it would at least make the iMac more 'sellable' to help pay for a new machine. It ended up being just the trick. The machine now feels as fast as it did with Snow Leopard installed.
I've upgraded many Macs and this was one of the easiest. A newer iMac might have been trickier with the front glass glued in place, but the suction cups opened this one up as easily as a G3 tower. Some people said suction cups weren't necessary, but they were totally worth it. The whole thing went so fast I forgot to take any pictures! My only obstacle was the T6. I thought I had one, but it was a T5. I only needed it for two screws so Murphy's law said one of them was the screw that would be nice and tight. I ended up sacrificing my T5 by filing the tip of the tapered bit off, essentially making it a T6/T7-ish bit (T6.5?). After that everything went very smoothly. I was upgraded, reassembled, and restoring from a Time Machine backup in about 20 minutes. Counting clearing off my desk and the T6 obstacle, the whole thing took about 40 minutes.
I had an extra pill case with a compartment for each day of the week. I used it to hold the screws as a went along with one compartment for each step. Made it much easier to go backwards for reassembly. I also worked with the iMac on it's back and the top of the screen facing me. I think this actually made it easier to remove the hard drive this way by just bracing my fingers and using my thumbs the release the drive. Of course, it also meant all the pictures in the guide were 'upside-down,' but I managed to find a everything. Having the guide on display on my iPad really helped.