Thank you to all for your answers.
I think, i will use the safe approach : i will clone my existing Golden Win 7 image in Virtual Box.
With the cloned image, i will install Ubuntu with Grub and upload the new “Golden Dualboot Win7/Ubuntu image”.
In my ref image, i do … nothing 'cause all conf", softs and scripts are run at the post-deployement …
I will try this approach and “i will be back” with the results.
Thank you for “the right way” 😉