According to my chat with DJI directly, they stated that all they need is a receipt. They call it "bill of sale" to honor the warranty. I am not the authoritative source though, so YMMV. I have the text saved off for proof. I did this because I was looking to buy used from the boards. Just FYI.A link would be helpful.
Remember - you want to buy from a DJI approved dealer for warranty.
Here's link to the screen captures from the DJI chat session. I'd be interested to hear what @msinger has to say.
DJI Chat - Transfer Warranty Buying Used - Chat - Buckeye Photography, Inc.
I decided to post it here in case it helps anyone. PLEASE DO NOT TAKE THIS IS THE ABSOLUTE TRUTH. That was just my experience and chat. Not sure if it would hold if push came to shove with DJI. Not a lawyer so, not even sure if this can be enforced legally. Just saying. If you buy used and has warranty left, don't blame it on me if DJI tries to get out of it. ;-)Good to know so I'm safe to still have some coverage if it ever works.