You may run into problems if you don't change your ticket. You may not be able to check in at all because the computer sees you have a ticket that violates MCT, or if you try to check a bag, they might not let you do that. So I would you say you really need to change your ticket. I can think of three options, none of which is great for you:
1. Get a refund for the ticket entirely and book on another carrier.
2. Can you fly into MKE instead? If there's an MSP-MKE flight that meets MCT, you may be able to convince a phone agent to book you on that instead.
3. Cross your fingers and hope for a another schedule change that makes your itinerary meet MCT again. Just don't wait too long for DL to have another schedule change or you may be forced to buy an expensive flight on another carrier.