They all are installed in each separate folder. The latest is set by the last install.
Supose you use 9.3 as working you may still export to old 2005 in ASCII. But you may just change to the folder where it is, e.g.:
C:\MentorGraphics\Pads2005\SDD_HOME\Programs and run either Layout, Router or Schematics from there.
The other method is running the configurator and select what you want, e.g.: Pads, EE, etc...