As far as I know, there is no supported procedure by CA to migrate UIM data from MSSQL Database to MySQL.
It is strongly recommended to do new UIM installation against MySQL server and migrate the collected QoS data. To do this, make sure to export/import the following tables before inserting QoS data into the new database:
S_QOS_DEFINITION
S_QOS_DATA
all RN_QOS_DATA_% tables