Article: UniBeast: Install OS X Yosemite on Any Supported Intel-based PC
I was able to update from Mavericks to Yosemite, I did not have any issues.
The steps I followed:
1. setup the BIOS per the TonyMac instructions. see below
2. Back up my OS (just in case I run into any issues).
3. download and created Unibeast per TonyMac instructions
4. Booted to the USB drive and installed Yosemite. I did not partition the hard drive.
5. Stepped through the installation. Used the Unibeast thumb drive to boot into OSX Yosemite, installed all the drivers I needed and rebooted the machine. The machine came right up and I logged in.
Now, the particulars of my machine are as fellows
Gigabyte GA-Z77X-UD5h rev 1 and bios f14
Intel Core i7-3770K Ivybridge
Nvidia GTX 660
16 Gigs of RAM
Bios settings, in my case I have a Win8.1 64bit partition that I run 64 bit VMs on so I did not disable the virtual settings in the bios, I personally have not had any issues with this and I believe from what I've read you only need to do so if you have a non K CPU
Using Multibeast I used the DSDT free option, installed only the drivers I needed, Realtek 898 for audio and AppleIE1000e V3.1.0 for network. I ejected the thumb drive rebooted the machine and it came up. Only issue I had was no sound. To fix that I opened System Preferences >Sound and changed Sound Effects and Output to "Internal Speakers"
Things I did not do. Use the USB 3, I have left those ports empty because in the past I had issues with waking, my keyboard and mouse would not come back. So far its always worked fine with out it. The iMessage, I couldn't tell you if it works or not as I do not have an iPhone so its never been an issue to me.
I hope that helps a bit to answer the question if you can upgrade. Its working pretty nicely for me.