Which initial DL-MCS and MCS Families you had before MCS change? And now (MCS and Families)?
Is Dynamic Link Adaptation Mechanism implemented in your network?
If it is, then what BLER (Block Erasure Ratio or retransmission rate) threshold values you have for downgrade switching points between MCS9 and other MCS (MCS9 -> MCSy). These thresholds are varied for different switching points.
Have you a BLER threshold that is lower than it was before change?