This subchapter is intended to permit only the refunding of valid outstanding bonds and accrued matured interest on the bonds of any municipal improvement district, expressly including bonds on which judgment has been rendered in any court and not including any other form of indebtedness, and then only after the board of improvement thirty (30) days prior to the actual refunding has given notice to do so in some newspaper of bona fide circulation in the county where the district which will refund is located.