poor performance after IDS9.30 upgrade

Hallo! After upgrading to IDS9.30FC3X3 I got a performance problem on a query. I haved analysed the sqexplain.out file, and I saw, that that the query optimizer uses a other index. update statistics runs every day for this database. After a reorganization of the indizes, the query optimizer chooses the wright index. Is it necessary to reorganize the indexes after informix upgrade? Are there any possibilities to check the indexes ( onchecks, sysmaster queries, .. )? Is it possible to check whether the indexes are often used? Thanks Wolfgang