The new MacOSX is a free download from Apple, and FlowJo v9 and FlowJo X both are supported.
I installed Yosemite on my 2013 iMac without issue. Note it is about a 6GB download, so allow a couple hours, then another 30 minutes or so to install it.
It seems a little faster to me, and Mail has some nifty new features. One can now send very large attachments since it integrates nicely with iCloud drive, and one can draw/markup any included graphic. We like these features in FlowJo support! The Icons are all a little different, but I am getting used to them.