When modifications are made to share-based payment awards, the appropriate accounting treatment depends on whether the modification is considered favorable or unfavorable to the employees. According to the principles outlined in the reference materials, if the modification is advantageous to the employees, such as an increase in the number of shares or a reduction in vesting conditions, the entity should recognize the impact of the modification that increases the fair value of the equity instrument over the remaining vesting period.

On the other hand, if changes to the terms of the equity awards result in a decrease in their fair value, the decrease in fair value is not necessarily recognized over the remaining vesting period; instead, the decrease in fair value typically doesn't lead to an immediate reversal of previously recognized expense. The entity would continue to recognize the expense based on the original grant date fair value of the original equity instruments, without considering the decrease in fair value resulting from the modification.

Therefore, the statement "When changes to the terms of existing equity awards appear to cause a decrease the fair value of the instrument, previously recognized expense should be reversed immediately" is not accurate according to the principles guiding the accounting for share-based payment awards. The correct approach would be to adjust the expense recognition for the period after the modification, reflecting the increased or decreased fair value, as appropriate, over the remaining vesting term.