"Operating Airline"
bmi is already the operating airline for EMA-BRU and NCL-BRU.
Currently the operating airline for BRS-BRU is SN Brussels, but they wetlease a bmi aircraft to operate it. From the 29OCT, bmi will become the operating airline for BRS-BRU.
bmi and SN already have a mutual codesharing agreement, ex-UK. That means the current BRS-BRU route has a bmi codeshare. As of 29OCT that will become an SN codeshare. It'll be the same aircraft and crew, but working for bmi, not SN.
As the "operating airline" (bmi) is not a member of Star Alliance, then Star Alliance benefits (like earning/redeeming miles, lounge access, through tagged bags, fast lane security, additional bags for status customers) will end.
Another side effect of bmi becoming operating airline for BRS-BRU, is that although the SN-BM codeshare arrangement will continue, the SN-LH codeshare arrangement will end, so unless BM re-instate that codeshare with LH directly, it will no longer be possible to book BRS-BRU as part of a LH itinerary. (The BRS-FRA/MUC flights will continue to be BM operated with LH codeshares).
For me, as a Star Alliance Frequent Flier, and a Lufthansa flyer, the change of operating airline from SN Brussels to bmi is going to be quite a negative change for BRS.