Depending on how you install it, you would have both RH11 & RH2015 installed – if RH2015 “expired” then you would still have 11 there; however, if you converted your RH11 projects to RH2015, there’s no going back to RH11, so this sort of “test” isn’t advised. Better to install the trial on a separate non-production machine & play with it using copies of your RH11 projects (not the real ones). Don’t make any changes to the “play” copies unless you are ok with losing the changes if you “expire”.
Thank you Jeff!
Just to note that as long as you understand the risks, there is nothing wrong with installing the eval alongside an older version. The main issues are as Jeff mentioned, don't open any existing work because you cannot revert. So make a backup copy and work with the backup.
And if you are a person that likes to double-click the XPJ to open your project in RoboHelp, because the newer version has been installed, it replaces the Windows file association so that double-clicking causes RoboHelp 2015 Release to open instead of the older version. So that may trip folks up.
As long as you only work with copies of projects and you begin by first opening RoboHelp, you should be fine using one PC.