Can a macrophage differentiate back into a monocyte?
No, macrophages can not turn back into monocytes, usually once a monocyte travels around the bloodstream for 1-3 days, it then leaves the bloodstream and travel to a tissue where it will differentiate to a macrophage or dendritic cell. It undergoes a conformational change, this usually happens during inflammation as the macrophages travel over a chemical gradient, macrophages can still enter the bloodstream, however, they won't re-differentiate to a monocyte. So to answer your question directly, macrophages don't stay in the same tissue forever as their life span is pretty long(1-2 months) so most likely an inflammation will occur and force the macrophages to diffuse across the chemical gradient, but they usually patrol around the tissue area where they differentiated.

source: http://www.histology.leeds.ac.uk/blood/blood_wbc.php