If the crack isn't serious enough to risk the flight integrity, I would fly it until it's 11mo old, keeping an eye on the crack size, then send it in for replacement. I personally have received 3 brand new craft as replacements for warranty problems that were easily seen or duplicated, I didn't get a refurbished craft. My P4 was 362 days old and I found a crack around the battery compartment. Other craft has show that location to be a common problem. DJI sent me a brand new craft with stickers, pristine as new. Maybe with
P4P they have changed that practice, I'm not sure. I have heard others getting refurbs as warranty replacements, and they aren't 100% functional, in which case you ask them for a prepaid shipping label to take it back, providing them a list of the problems. I think it helps if your correspondence sounds like you're a fan boy, loving DJI, etc. That's what I do.