Like I say, there is no plan to remove Assad: that is what the rebels have been trying to do for 7 years. Initially we tried to rein in his early crimes against humanity, before all out civil war broke out, but were thwarted in that by Russia (and Chine) in the UNSC. Probably what we should have done, while we had more influence in the area, was make it plain to Russia that we had "no selfish or strategic interest" (to borrow a phrase) in Syria, or at least that we had no desire to break up Syria's alliance with Russia. That might have worked, it might not.
For a while we supported selected rebel groups, but with the rise of ISIS (aided and abetted by Assad ironically initially) a whole new front opened up. A key point was in 2013 when Obama and Cameron failed to act on the crossing of the "red line" in the use of chemical weapons. Russia stepped in as a bogus (as it turned out) intermediary, pretending to decommission all of his chemical weapons, and while we patted ourselves (and them) on the back, Assad stepped up his murder by more "conventional" means.
When Russia stepped into a military vacuum along with Iran, the course of the war changed. And the (welcome) defeat of ISIS has actually complicated the situation elsewhere further.
Getting rid of Assad now would not be very sensible, even if it were possible. I think the idea is to be seen to be taking a stand against his continued use of chemical weaponry, but I have no real idea how they intend to take deterrent action: plainly a lot more thought is going into it, this time, which is welcome.