You were all right! They "misinformed" me the first time I called. Call 1, they said 50%. Chat session followup, they said 100%. Then, Call 2, they told me that yes, I would have to pay 100% of my new Pixel XL before upgrading again. For sure.
I told them that they just removed any incentive for ordering through them, as 1.) a phone with a locked bootloader might not hold its resale value as well, and 2.) I don't need to be locked into
any type of a contract if I can bring a phone to them.
I let them know that since this was different from the Note 4 that I'd had on my last device payment plan, they should have TOLD me about this significant change... oh, well - heading to the back of the line.
I could have left the order stand and paid in full when it arrived, but for what? If I can't trade up at 50%, why buy the phone with the locked bootloader that will likely not hold its resale value. And... the whole thing just irritated me.
I so appreciate the OP and everyone here!