Is it possible to downgrade the 4900M IOS image from Universal 15.x image to non-Universal (possibly IP BASE) 12.x image. If possible, how can this be done and what will the feature differences be?
Don't understand WHY you want to do this.
Can you post the output to the command "sh license"?
If your permananent license is IP Base then downgrading the IOS from a Universal to "IP Base" is just a waste of time when features are exactly the same.