No guarantee
Buying a M or higher doesn't necessarily mean you will get the upgrade. I'm on a BOS-MSP flight currently, M Class, sitting in coach. Was #5 on the upgrade list. They wouldn't even let me upgrade using miles. If I didn't live in Atlanta, I'd certainly drop Delta. Luckily I'll be Gold in a few more flights. They certainly have flipped the finger to the Silvers recently.