Installing El capitan with differente Apple Id

Hi everyone,

I have a customer with an iMac 2009 running OS X 10.6.8. I want to install on it the new OS X El Capitan. The problem is my customer doesn't remember the Apple Id & Password associated with this iMac, he don't even remember the email account used to create the Apple Id so I thing is impossible to recover that Apple ID.

My question is if its possible to create a new Apple Id and use it to download and install the new OS X without lose or mess up all the data and applications actually running on the iMac

TiA