<PAGE>   1
                                                                   EXHIBIT 10.58


                               FIFTH AMENDMENT TO
                 DYNEGY INC. PROFIT SHARING/401(k) SAVINGS PLAN

         WHEREAS, Dynegy Inc. (the "Company") and other Employers have
heretofore adopted the Dynegy Inc. Profit Sharing/401(k) Savings Plan (the
"Plan") for the benefit of their eligible employees; and

         WHEREAS, the Company amended and restated the Plan on behalf of itself
and the other Employers, effective as of January 1, 1998; and

         WHEREAS, the Company desires to further amend the Plan on behalf of
itself and the other Employers;

         NOW, THEREFORE, the Plan shall be amended as follows, effective as of
October 1, 1999:

         1.       Section 8.3 of the Plan shall be amended by adding the
following new Paragraph (g) to the end of such Section:

                           "(g) Paragraphs (b), (d), and (e) above
                  notwithstanding, if a Member shall cease to be employed during
                  the one-year period beginning on the date of the closing of
                  the Illinova Transaction by reason of an Involuntary
                  Termination (as such terms are hereinafter defined), then such
                  Member shall have a 100% Vested Interest in his Employer
                  Contribution Account. For purposes of this Paragraph (g), the
                  following terms shall have the meanings set forth below:

                           (1) 'Excluded Termination' shall mean any termination
                     of a Member's employment with the Employer by reason of a
                     merger, acquisition, sale, transfer, outsourcing,
                     reorganization or restructuring of all or part of the
                     Employer or any affiliate or division thereof where either
                     (i) such Member is offered another position within the
                     Employer that provides such Member with a base salary at
                     least equal to or greater than his base salary in effect on
                     the last day of such Member's active service for the
                     Employer (including, without limitation, a position that
                     would require such Member to transfer to a different work
                     location so long as he has been offered the Employer's
                     standard relocation package in connection with such
                     transfer) or (ii) such Member accepts any position with a
                     successor company (as hereafter defined), including an
                     outside contractor, whether affiliated or unaffiliated with
                     the Employer. For purposes of the preceding sentence, a
                     successor company is (A) any entity that assumes operations
                     or functions formerly carried out by the Employer (such as
                     the buyer of a facility, asset or division or any entity to
                     which the Employer operation or function has been
                     outsourced), (B) any affiliate of the Employer, or (C) any
                     entity making the job offer at the request of the Employer
                     (such as a joint venture of which the Employer or an
                     affiliate is a member).

                           (2) 'Illinova Transaction' shall mean the
                     transactions contemplated in that certain Agreement and
                     Plan of Merger dated as of June 14, 1999, by and among
                     Illinova Corporation, Energy Convergence Holding





<PAGE>   2


                  Company, Energy Convergence Acquisition Corporation, Dynegy
                  Acquisition Corporation, and the Company, as the same may be
                  amended from time to time.

                           (3) 'Involuntary Termination' shall mean, with
                  respect to each Member, any termination of such Member's
                  employment with the Employer; provided, however, that the term
                  `Involuntary Termination' shall not include a voluntary
                  resignation by such Member, an Excluded Termination, a
                  Termination for Cause, or any termination as a result of such
                  Member's death or total and permanent disability (as defined
                  in Section 7.2).

                           (4) 'Termination for Cause' shall mean any
                  termination of a Member's employment with the Employer by
                  reason of such Member's (i) conviction of a misdemeanor
                  involving moral turpitude or a felony, (ii) engagement in
                  conduct which is materially injurious (monetarily or
                  otherwise) to the Employer or any of its affiliates
                  (including, without limitation, misuse of the Employer's or an
                  affiliate's funds or other property), (iii) engagement in
                  gross negligence or willful misconduct in the performance of
                  such individual's duties, (iv) willful refusal without proper
                  legal reason to perform such individual's duties and
                  responsibilities, (v) material breach of any material
                  provision of any agreement between the Employer and such
                  individual, or (vi) material breach of any material corporate
                  policy maintained and established by the Employer that is of
                  general applicability to Members."

         2.       As amended hereby, the Plan is specifically ratified and
reaffirmed.

         IN WITNESS WHEREOF, the undersigned has caused these presents to be
executed this _____ day of ________________, 1999.

                                                  DYNEGY INC.


                                                  BY:
                                                     ---------------------------
                                                      NAME:
                                                            --------------------
                                                      TITLE:
                                                            --------------------



                                      -2-