Funding or refunding bonds of a county, city, town, township, board of education or school district issued under existing laws may not bear a greater rate of interest than the indebtedness which is funded or refunded, or may bear such rate or rates of interest which shall not constitute a greater overall interest cost existing on indebtedness which is funded or refunded. Provided that no bonded and/or interest coupon indebtedness shall be refunded except such as have been issued and outstanding at least one (1) year at the time of such refunding.
Okla. Stat. tit. 62, § 357