So the basic theory is to create a second bootable partition on your disk (if you only have one like I do) or one another disk if you have several. My system disk is 1TB but not especially full so there was room to create a 100GB play partition for Yosemite. The procedure is documented in a set of web pages mostly on osxdaily.com. The most important thing is a full backup before the partition resize operation and actual installation. The installation I did from a USB installer created from the downloaded Yosemite upgrade. I suspect you could live without this step but later on if the test is successful and you want to do the upgrade for real you can save yourself a download. Well except that by then 10.10.3 might be out and you might want that. In any case I ended up creating a dual boot USB drive that would boot to either the Mavericks or Yosemite installer. As a bit of background it is worth understanding how to downgrade if anything goes horribly wrong, though in this case with a second partition you can pretty much just destroy the partition and start again. But I looked at how to downgrade via TimeMachine if required. I haven't had to do it so I cannot speak to its efficacy but the process is apparently very easy. Finally there is a Mac Tips description of how to do a dual boot Mavericks Yosemite setup. Most usefully it shows you how the Startup Manager looks if you've not used it before, which I hadn't.
The basic upgrade safely procedure is here : http://osxdaily.com/...emite-update/ [UPGRADE]
The procedure for creating USB boot drives from the installation software is here : http://osxdaily.com/...-install-drive/[USB]
Partition resizing using Disk Utility is very simple even with data in the partition : http://osxdaily.com/...h-disk-utility/[DISK UTILITY]
Downgrade procedure : http://osxdaily.com/...to-mavericks/ [DOWNGRADE]
Dual boot of Yosemite Mavericks : http://mac-how-to.wo...10-10-0155422/ [DUAL BOOT]
0. Read all the above first before doing anything. This step is really important.
1. Download the Yosemite installer from the App Store. It will automatically start running and present you with the first splash page. Just go to the menu and exit it. You will find the /Applications/Install OS X Yosemite.app. You need this file later so remember where it is.
2. Get a USB stick that you can erase or repartition. There is a little bit of confusion in some posts about how big this drive needs to be. Both Mavericks and Yosemite create a bootable installer of about 5.3 GB, with Yosemite being ever so slightly smaller. So I created a two 8 GB partitions on a 32GB flash drive so I could do both. But if its just Yosemite you'll only need one. Follow the steps in [USB] above to do this.
3. At this point do a full backup of your system. I did this with TM. See [BACKUP] on how to get an immediate TM backup done. I actually dropped in a new USB drive for other reasons, into TM and it pretty much decided to do an immediate first backup which is always a full one, so that was simple. In any case get one. TM is a good choice because [DOWNGRADE] above shows you how to use it to get yourself out of trouble if necessary. I always like not only to have a backup but also to know how to use it before that becomes necessary. Researching how to do recovery while your Mac is in critical condition is not as entertaining as it sounds.
4. After the backup is done, you can repartition your system disk as per [DUAL BOOT] above. I created a 100GB partition so I had lots of room for test apps and so on but it's really way too much for most purposes.
5. Then load up the USB boot installer and restart the Mac holding the option key. I am not a Mac expert and don't know exactly how when the key is checked for but the simplest way I found on my iMac to do this reliably was to do a Shutdown, not a restart, and then hold the option key down while hitting the power switch to boot the Mac. Then once the Startup Manager appears (again see [DUAL BOOT] for a screen shot, you use the arrow keys - yep no mouse support at this point to select the OSX Installer. From here on it's just a straightforward fresh installation of Yosemite. Make sure you select the new Yosemite partition you created when you choose where to install !
6. This seems obvious, but do not rush through this. It will take some time, the back can literally take hours, but don't short cut it.
7. Switching back and forth between the systems is another shutdown, option-startup cycle but that's it.