<PAGE>   1

                                                                     EXHIBIT 4.6


                                 AMENDMENT NO. 1

                                       TO

                    RESTATED AMVESTORS FINANCIAL CORPORATION

                       1989 NONQUALIFIED STOCK OPTION PLAN

          WHEREAS, AmerUs Annuity Group Co., f/k/a AmVestors Financial
Corporation, a Kansas corporation (the "Corporation"), heretofore has
established and maintained the "RESTATED AMVESTORS FINANCIAL CORPORATION 1989
NONQUALIFIED STOCK OPTION PLAN" (the "Plan"), an incentive stock option plan for
the benefit of certain of its employees and certain employees of its affiliates;

     WHEREAS, effective December 19, 1997, the Corporation merged with and into
     AmerUs Group Co., f/k/a AmerUs Life Holding, Inc., an Iowa corporation
     ("AGC"), pursuant to which each outstanding share of the common stock of
     the Corporation became 0.6724 shares of AGC stock for every share of their
     Corporation stock (the "Merger"); and

          WHEREAS, the Corporation desires to amend the Plan to reflect the
assumption of the Plan by the Corporation and the substitution of Corporation
common stock for AGC common stock in the Plan's company stock fund.

          NOW, THEREFORE, the Plan is hereby amended, effective December 19,
1997, as follows:

          6.   The title of the Plan shall hereinafter be the "RESTATED AMERUS
               ANNUITY GROUP CO. 1989 NONQUALIFIED STOCK OPTION PLAN."

          2.   Section 1 of the Plan is hereby amended and restated as follows:

     "1.   Statement of Purpose. The purpose of this Restated Nonqualified
Stock Option Plan (the "Plan") is to establish and continue as close an identity
as is feasible between the interests of AmerUs Annuity Group Co. (the
"Corporation") and its subsidiaries and the Directors, officers and employees of
the Corporation and its subsidiaries (the "Eligible Participants"). The Plan is
intended to encourage a sense of proprietorship on the part of the Eligible
Participants, who will be largely responsible for the continued growth of the
Corporation and its subsidiaries, to recognize past valuable services of such
Eligible Participants, to furnish such Eligible Participants with further
incentive to develop and promote the business and financial success of the
Corporation and its subsidiaries, and to induce such Eligible Participants to
continue in the service of the Corporation or its subsidiaries by providing a
means whereby such Eligible Participants may be given an opportunity to purchase
shares of common stock of the Corporation's parent company, AmerUs Group Co., an
Iowa corporation (or any successor corporation) ("AGC Stock"). The Plan will
also serve to effectuate the Corporation's intention of granting options to
certain Eligible Participants in substitution for certain options originally
granted to such Eligible Participants under (a) the Corporation's old Incentive
Stock Option Plan, adopted August 14, 1986 (the "1986 Incentive Stock Options");
(b) the Corporation's old Nonqualified Stock Option Plan, adopted August 14,
1986 (the "1986 Nonqualified 'Stock Options"; and (c) the resolution adopted by
the Board of Directors (the "Board of Directors") of the Corporation on August
14, 1986, pursuant to which certain options to purchase shares of common stock
of American Investors Life Insurance Company, Inc. were converted into options
to purchase shares of common stock of the Corporation (the "American Investors
Options") (collectively the "Exchanged Options")."

          3.   Section 2 of the Plan is hereby amended and restated as follows:

     "2. Administration by Board. The Plan shall be administered by the Board
of Directors. The Board of Directors shall be the sole determinant of which
Eligible Participants may be granted options to purchase shares of AGC Stock
under the Plan, how many shares each Eligible Participant may purchase, the
price to be paid for the

<PAGE>   2

shares, the terms of payment, the option period, and any interpretations of the
Plan's intent and operation. In administering the Plan, the vote of a majority
of the Directors voting, provided they constitute a quorum under the
Corporation's bylaws, shall be conclusive."

          4.   Section 4 of the Plan is hereby amended and restated as follows:

     "4. Availability of Stock. A total of 1,405,970 shares of AGC Stock shall
be available to the Plan. These shares may come from authorized but unissued
shares and from issued and reacquired shares, including shares issued in
accordance with the Plan and reacquired by the Corporation. Shares of AGC Stock
available to the Plan may be subject to any restrictions placed on them by the
Board of Directors in accordance with the terms of the Plan or the Nonqualified
Stock Option Agreement described below. The total number of shares available to
the Plan shall be adjusted to account for stock splits, stock dividends, reverse
stock splits, granting of warrants, and the like."

          5.   The first paragraph of Section 7 of the Plan is hereby amended
               and restated as follows:

     "7. Manner of Exercise. The payment of the option exercise price of an
option hereunder shall be either in cash or, if the Board of Directors, in its
sole discretion, so specifies, through delivery to the Corporation of shares of
AGC Stock with an aggregate fair market value equal to the option exercise
price, or by any combination of cash and shares (if the Board of Directors has
approved payment through delivery of shares). The fair market value of shares
delivered as payment, in whole or in part, of the option exercise price shall
be:"

          6.   Section 9 of the Plan is hereby amended and restated as follows:

     "9. Stock Reserve. The Corporation at all times during the term of this
Plan shall reserve and keep available such number of shares of AGC Stock as will
be sufficient to satisfy the requirements of this Plan, and shall pay all fees
and expenses necessarily incurred by the Corporation in connection with the
exercise of options granted hereunder."