Two or more districts having a single appointing government may, by joint petition filed with the appointing government, request that the districts be merged. If the petition is approved by the appointing government, the districts shall be merged into a single district. No merger shall alter any existing assessments or bonds nor shall a merger result in any existing assessments being pledged to bonds to which assessments were not pledged prior to the merger. No merger shall impair the security of any outstanding bonds of the district.