Downgrading from Master to HiFi Quality will result in Tidal converting the MQA album to 16/44.1. If it was originally a 44.1kHz-multiple 24-bit album, the 8 bits are simply truncated resulting in a 16/44.1 MQA CD. If it was originally a 44.1kHz 16-bit MQA CD such as early Madonna albums, it will remain to be the same MQA CD. If it was originally a 48kHz-multiple album, I believe the MQA signaling is destroyed in the conversion to 16/44.1.
From this perspective, the difference between a HiFi non-Plus and HiFi Plus account is that the former will force the downgrade of Master quality to HiFi 16/44.1 quality.