Statement A is not entirely accurate because the entity does not record both the increase in fair value from the modification date and the amount based on the grant date fair value; rather, the entity would recognize the change in fair value only to the extent that it impacts the service received from the employee.

Statement B is partially correct. If changes to the terms of existing equity awards result in a decrease in fair value, the decrease is generally not recognized over the remainder of the vesting period but may impact the accounting for the modified award going forward.

Statement C is not correct. Previously recognized expense is not reversed immediately just because of a decrease in the fair value of the instrument; the expense would typically continue to be recognized based on the original grant date fair value, without adjusting for the decrease in fair value.

Accordingly, the most accurate statement in this context would be:

B. When changes to the terms of existing equity awards appear to cause a decrease the fair value of the instrument, the decrease is recognized over the remainder of the vesting period.

This aligns with the principle that modifications to share-based payment awards that result in a decrease in fair value do not lead to an immediate reversal of the previously recognized expense. Instead, the expense recognition continues based on the original grant date fair value.