(buy-sell agreement entered into before Oct. 8, 1990, not subject to valuation restrictions of I.R.C. Sec. 2703; such agreements not subject to I.R.C. Sec. 2703 if not substantially modified after Oct. 8, 1990; here, agreement modified to extend repayment term and IRS viewed extension as merely de minimis change to quality, timing or value of rights of parties to agreement because agreement required reasonable interest rate to be paid; agreement also modified to specify that "prime rate" is rate to be adjusted semiannually, and this was also not substantial modification because resulting payments would more closely approximate fair market value; requirements of Treas. Reg. Sec. 20.2031-2(h) must still be satisfied to control valuation issues at death).