Estate Tax Valuation Erroneous

I'm reviewing a return where an attorney, acting as the executor of an estate, used the services of an appraiser to value farm land at what appears to be a value substantially below fair market value. I have yet to determine if this was done intentionally to avoid state taxes but, it is more than likely simply due to an error in valuation. The numbers are signification ($400K versus $800K). We know this because there have been two offers by two different sellers for $800K. My question is this: if it turns out that the valuation was simply wrong, can an amendment be done to the estate tax return and revaluate the property or is there something under Plaut v. Munford, 188 F.2d 543, 545 (2d Cir. 1951) where an appraisal for estate tax purposes is "prima facie" evidence of the property when the income tax is calculated for a subsequent sale. In other words, could I just take a position on the tax return as long as I have evidence to support the corrected fair market value? FYI, the sale of the property did NOT occur within 6 months of close of the estate. Thanks for any input you may have on this matter.