1. CPO does NOT 'require' an oil change. However the CPO procedure DOES requrie they do any indicated maintenance. Here is a question: Was the oil supposed to be changed at the time of CPO? By BMWs estimation, not yours. In other words does BMWs records show a lapse in maintenance?
2. If no, you have no recourse.
3. If yes, the dealer has exposed you to a potential claim that you failed to maintain the car and the warranty is void.
They won't do a damn thing for you though.
Now, sounds like you are at 20 months and 13k miles? Should be OK. Personally I think that is too many miles, but BMW is OK with it. I also think they've stretched the interval to two years.
Just change it, move on. And check you oil level.
A