I just went through this last week. When you attempt to install V12, you'll get a prompt to uninstall V11. Decline to have it unintall/update the files and it'll install V12 by itself. I found that after V12 was installed, V11 no longer worked. To correct that, there is a copy of V9 & V11 on the V12 CD. I reinstalled V11 from the new CD it worked like a charm.
I also had V9 on my PC and that seemed to work fine after the V12 install. I didn't have a need to reinstall that version.