The main reason for bombing the hell out of Assad's military is not regime change.
The USA is going to do this to show the world that the USA means what it says when it makes a threat.
President Obama warned Syria that if it used chemical weapons against its people that there would be consequences. What is about to happen in Syria will be a taste of those consequences.
If the first attacks don't get the job done there will likely be more until Assad gets the message.
The USA will not be taking this on unilaterally, it will be supported by it's allies who agree that in the 21st century chemical wepons can't be used with impunity and that any country that uses them as Syria has done in this case must pay a heavy price.