In many of our combats, some portion of our enemies run or teleport away to where we cannot engage them further. Our warlock has become concerned about effectively losing a valuable spell slot, possibly long before the combat is over.
The spell description states that you can retarget when the current target drops to 0 hit points. That is stated as a sufficient condition, not a necessary condition. It was not stated as "Only if the target drops to zero hit points...". It does not say that you can't retarget anytime you want. Usually the authors are very specific about what you can or cannot do, so for this spell it would seem that you can retarget anytime you want. My interpretation is that the authors are simply letting the players know that when the target drops to 0 health, the spell does not end.
I have tried to find an official ruling via Sage Advice and the PHB errata with no success. I was hoping that someone would have a link to an official comment on this topic, either from an original author or someone from WOTC.