It seems like you simply need to reverse the procedure you give. Since you initially had to pull the rope up far enough to tie the stick in to it, you can still pull it back up to the same point in order to untie the stick. Then let the bag lower down again, stick free.
Here is my hastily doodled interpretation of the process. The stick is represented by the red mark in each drawing. There is at each step, more rope than is required. At step 4, the cueball figure reaches up to tie the stick into the rope, but still leaves an ample amount of rope hanging at his feet. In step five he lowers the bag to this stick, then in step 6 he secures the loose rope around the tree trunk.
To lower the bag, reverse the steps. Take the loose rope off the trunk, raise the bag until you can again reach the stick. Remove stick, lower bag, undo everything, make breakfast, resume hike.