If you have BCBS you either have a Medicare Advantage program managed by BCBS or a Medicare Supplemental from BCBS? If you were only on Medicare, BCBS wouldn’t be involved in approvals.
Pure Medicare might have better coverage than a BCBS managed program. I’ve had to sift through multiple providers and Medicare coverage for myself, husband, and sister. She used to use Medicare through Aetna. My husband used to use Medicare through UHC. Each have different coverage, and are different than choosing to just use Medicare alone.
It could be that BCBS is the coverage that is limiting your options. As I understand it, Medicare alone continues to pay for breast cancer follow ups, BC related breast changes, and scans.

If you want to make a change to a medicare advantage program you still can. Although I would check to see that there were no penalties, or increase in Medicare costs before doing so. These shouldn't be a problem, but it depends on individual circumstances.
“The main period to change Medicare Advantage plans is the Medicare Advantage Open Enrollment Period (MA OEP), running annually from January 1 to March 31, allowing one change to another MA plan or back to Original Medicare.”