Good news/bad news. I know I've tried installing LabVIEW older versions after newer ones, and it never worked (this is the Bad News, if you couldn't tell). When I got started and needed to do this (say, install LabVIEW 7 (again) with LabVIEW 8.0, 8.2, and 8.6 already there), the fastest (and only working) way was to get Control Panel to uninstall all NI Software and "start all over".
But there's a "better way" -- make VM for the older version. The nice thing about a VM is that it acts like a "PC as Software" -- you build a 60GB VM with Windows 7 x64 and a few tools, and save it as "Basic Win 7". When you need, say, LabVIEW 12, you copy the VM, open it with (free) VM Player, and go install LabVIEW 2012. If you have a PC with a little more "oomph" (say 16GB core and at least an I5, better a quad core I7), you can give the VM 6-8GB of memory and 2-4 cores, and have a not-so-bad machine. The downside, of course, may be hardware connectivity, but for development, this is hard to beat.
Bob Schor