"If the company retains the right to unilaterally amend the plan to reduce or eliminate deferrals and/or accruals ... only the amounts deferred prior to January 1, 2018, would be excluded [from the $1,000,000 deduction limitation]. This becomes important because of the change under TCJA to the definition of 'covered employee,' from those employed in certain positions during the year to a 'once covered always covered' approach."