1. A non-transitory computer-readable medium storing therein a data conversion program that causes a computer to execute a process comprising: receiving after a schema of a database has been changed from a former schema to a new schema, a processing request concerning the database; judging based on difference information concerning the former schema and the new schema, whether in the processing request, a condition that specifies process data to be processed has been changed by the new schema; creating, when judgment results obtained at the judging indicate that the condition has been changed by the new schema, a search expression by converting the condition into a condition that corresponds to the former schema; searching the database for conversion data whose format is to be converted from the former schema to the new schema in accordance with the created search expression, the searching based on the judgment results obtained at the judging and on the processing request; and converting the format of the conversion data searched at the searching, from the former schema to the new schema, wherein the database stores therein data related to configuration items of a system and data related to relationships expressing associations between the configuration items, the process further comprises executing retrieval of data related to a second configuration item that is associated, by a relationship, with the conversion data that is retrieved at the searching and related to a first configuration item; and determining which data retrieved at the executing retrieval is to be subject to conversion, when data related to a plurality of second configuration items is retrieved at the executing retrieval, the determining based on index values indicating respective occurrence frequencies of the second configuration items, and the converting includes converting, from the former schema to the new schema, the format of the determined data and the format of the data related to the relationship expressing association between the first configuration item and the second configuration item indicated by the determined data.