I agree with Luke and I'm not a CPA either. However, the NASPP newsletter (Barbara Baska) has a good example of how accounting works if a partially vested option upon termination of employment is accelerated and becomes fully vested. It's not treated a a new grant as to the portion of the award that has already vested. It is a new grant as to the newly vested portion. Here's her example:
"Sa that an employee terminates while holding an award to purchase 10,000 shares that is 75% vested, and the company modifies the terms of the award to accelerate vesting on the remaining 2,500 unvested shares. Further assume that the fair value of the award at grant was $10 per share, and the fair value at the time the modification occurs is $16 per share.
The fair value of the vested portion of the option is $75,000 ($10 per share multiplied by 7,500 shares); this expense has already been recorded and is not reversed.
The original fair value of the unvested/accelerated portion of the option was $25,000 ($10 per share multiplied by 2,500 shares). Let’s say that the termination occurs after one-fifth of the last vesting period has elapsed, so that the company has already recognized $5,000 of expense for this tranche (assuming straight-line accrual and that the company accounts for forfeitures as they occur). The $5,000 of expense that has already been recognized is reversed and the remaining $20,000 of expense is never recognized.
The fair value of the unvested/accelerated portion of the option after the acceleration is $40,000 ($16 per share multiplied by 2,500 shares); this amount is recorded as expense in the period in which the acceleration occurs.