The ony way you can reduce a prior work log entry is to enter another negative one. You can enter a negative work time such as -30 and the system will accept it, and it is correctly subtracted from the total work effort. (I could only enter a negative # of minutes. -1d is not accepted by the system, but -480 is.) In any case, the negative entry is not shown in the work log entry.

In addition, the system will allow a negative net work effort.

If the entry of negative time is disallowed in a future release, will there be a valid way to back out work estimates?