your older games will run on the older dx versions anyway. remember each version is separate from the others so you likely already have multiple versions installed depending on the games you play.
having dx 12 installed does not mean an old game will use it. it will use the versions it was written on unless it is patched to use a different version. some dx11 games have gotten dx12 patches but not all of them. most developers won't bother unless it is a very new or very popular game to justify rewriting it for the new dx version.