From the Medicare webpage:
What it means to pay primary/secondary
The insurance that pays first (primary payer) pays up to the limits of its coverage.
The one that pays second (secondary payer) only pays if there are costs the primary insurer didn't cover.
The secondary payer (which may be Medicare) may not pay all the uncovered costs.
If your employer insurance is the secondary payer, you may need to enroll in Medicare Part B before your insurance will pay.
Paying "first" means paying the whole bill up to the limits of the coverage. It doesn't always mean the primary payer pays first in time. If the insurance company doesn't pay the claim promptly (usually within 120 days), your doctor or other provider may bill Medicare. Medicare may make a conditional payment to pay the bill, and then later recover any payments the primary payer should've made.
So from what I gather reading that, you insurance will continue to pay what it does now, and then Medicare *may* pick up some or all of the leftover amount. On the page it says if you have additional questions to contact the Medicare COB contractor. So you may want to give Medicare a call and see if they can give you more precise information.
In my experience, when Medicare becomes primary, you should have little to no OOP costs. Since my husband came on my insurance, my insurance has been paying 100% of what Medicare doesn't, up to the allowable amount. We've paid nothing on any of his claims. We still may hit our $5000 OOP though with all of our counseling appointments (which get billed through just my insurance) and the other stuff me and the kid have done this year. I only put $1000 in my FSA too
Going to have to rethink that for next year I guess.