Changing a monster's target will clear any patrol routes it's following, and will not set a new one; it stays cleared: https://github.com/ValveSoftware/halflife/blob/5d761709a31ce1e71488f2668321de05f791b405/dlls/triggers.cpp#L2123
There are 2 exceptions: Apaches and Ospreys will both reset their target entities to the current "target" keyvalue if it's null, so using changetarget on them will reset their paths.
I can implement a feature in HLE to make this set a new patrol route for any monster, but i doubt you're using that for your map.