I just bought a 2nd hand MacBook Pro, early 2011, which came with Lion. The previous owner upgraded it to Mountain Lion. AppleCare staff confirmed that a Mac that comes from Apple with Mountain Lion cannot be downgraded because the firmware is fixed to ML. That is official. AppleCare senior staff told me over the phone (in relation to another matter). Now, the question is, if a Mac came from Apple with Lion - and is upgraded to Mountain Lion - can that Mac be downgraded back to Lion? I searched for answers on the forum, but many of the comments seemed to be for pre-release versions of ML. In my case, I kept a copy of the InstallESD.dmg file for Lion, so there is no need to go into all the aspects of actually getting the Lion install file. I have that. I know the issues of not being able to access Lion from the App Store after upgrading. That all is irrelevant to me, since I kept the InstallESD.dmg, so please don't comment on that aspect. Just focus on the technical issue of whether Lion can be re-installed after Mountain Lion has been installed, particularly whether there are any consequences due to Firmware changes from Mountain Lion.