After downgrading, the Oracle standby connection will be broken as it is not able to do function string mapping or data translation. If you did not drop standby connection before downgrading the RSSD, you can drop it after the downgrade.
After you have completed downgrading the RSSD using the rs_init, your connection to Oracle data server (if your connection to Oracle is created using the create connection with using profile clause) may be down because the wait_after_commit configuration parameter provided in Replication Server 15.5 is no longer available, thus you need to resume the replication process.