Don't worry about the blue flames migrating towards the hopper. As long as the RED glowing coals don't migrate backwards you will be fine. To prevent the migration of the burning coals, make sure your draft maintains itself to recommended levels and if you remove the burn grates for cleaning, MAKE SURE you cement them back in the same way they were originally (this ensures the blowing air follows the proper path!!!!).
I posted the following on another earlier thread at : IS this flame too close to the hopper?
One of my responses to the previous thread : On my koker, the glowing part of the coal bed never moves upstream of the grate vents, but the blue flames will migrate (waft) towards the back, most likely just being blown around by the combustion air. The air is blowing through a good 2 inches of coal, so some of it will wander a little and not blow straight up.