After the refunding has occurred as provided in Section 33471, not less than 20 percent of all taxes which are allocated to the redevelopment agency pursuant to subdivision (b) of Section 33670 for redevelopment projects merged pursuant to this article shall be used by the agency for the purposes set forth in Section 33334.2, provided that such taxes shall first be used for the payment of principal, interest, and premium, if any, under the bond resolution or resolutions providing for the issuance of the refunding bonds and providing necessary reserves for such refunding bonds, but only to the extent that such refunding is necessary to refinance existing bonded obligations.