Adobe only supports one installed version of Acrobat on your system. When I say "supports", that does not mean that two you cannot install two versions, but if something goes wrong, you will not get any help from Adobe.
The default behavior of the installer is to remove any previous version, but you can override that during the installation. The only thing that will limit you in installing Acrobat 9 again is that there is already a newer version of Acrobat installed, so the installer for the earlier version will not let you continue, so you will have to remove Acrobat X again, then install 9, followed by X (and selecting to keep the previous version).
Again, if something goes wrong, then you are on your own.