From your profile, I see you are gold subscription customer, so you license has the cascading functions, you could use right now release version and three older release together. That means you have Maya 2012 subcription license right now, Maya2011/2010/2009 also could use this Maya2012 license. If you have standalone license, you need to contact Autodesk Businese Center for downgrade license. If you have network license, use it.
You could check the license you get whether has cascade function. If not, please contact Autodesk businees center to apply new subscription license.
Yong Jing Product Support Specialist Frontline Technical Support, CSS. GCSO