I would call the front desk at SSR and explain, then see what they say. They may be able to put a late arrival notation on your reservation...or it could be that the computer system would automatically cancel the booking on Monday night, regardless. But it never hurts to call and ask. Be sure you actually talk to the front desk manager and not the WDW operator, AKA the virtual front desk.
You should call MS they will handle the RCI exchange too. Once they have looked up the confirmation number they can put in the notes that you are arriving two days later.
I have done this before. I’d also call ssr a few before checkin to advice them about it. Just to be on the safe side.
Have in mind that you are still checking out on the same date, but only having a 5 days stay.