OK, so here's how the deal went down.
The word on the street (internet) about Comcast has been:
-the only 4K boxes are DVR boxes
-you're not allowed to just order one online; they have to be provided as part of an in-home tech visit where they evaluate your system to make sure it's suitable
-there's no additional monthly charge for 4K service
So then, I initiate an in-home visit the way they say you should: online chat with Xfinity person online or through their app. Here's how that went:
ME: I'd like to upgrade my DVR box to 4K
MONDOHAR (Xfinity rep): OK, I'll ship one to your home
ME: Wait, what? I thought you only handed them out with in-home service calls
MONDOHAR: Let me check...nope, says here I can just ship you one
ME: Kick ass, when can you get one to me?
Long story short, we decide I'll go grab one at the Xfinity store, avoiding shipping cost and getting one right away. Oh, and I did confirm that it's no extra charge per month.
Act II
ME: Mondohar sent me; here's my old DVR box
PAUL (Xfinity Store guy): I'll go grab your new box; by the way, did you know you're on an old plan?
ME: I did see the new plans in my billing statement; thanks for bringing it up
PAUL: I can triple your Internet speed and get you the same channels for $50 less per month [not an intro rate]
ME: I'm pickin' up what you're puttin' down
PAUL: You'll need a new gateway; I'll get you that now, and you can bring the old one back later
ME: It's a deal, my friend
I'd been having trouble with one of my non-DVR boxes, so I figured I was on a rare roll with Comcast and decided to take care of that problem, too.
Act III
ME: I'm back, Jack; I brought you not only my old gateway but also my balky other box
PAUL (again!): Sounds like a dead box alright; I'll switch it out for a wireless non-DVR box that's also 4K capable, just in case you get another 4K TV
ME: Wait, what? I thought the only 4K boxes were DVRs
PAUL: Shows what you know, libtard [no, he didn't actually say this]
ME: My gateway's two floors down from where this'll go; is that too far for wireless?
PAUL: Works in my old house with thick floors and plaster, so no prob
And sure as dang hell, that wireless box works like a champ. I haven't had a chance to seek out any Comcast 4K content yet, but I'll report anything noteworthy. But the bigger picture is that things went about 300% better than I expected: a mostly glitch-free process with Comcast that included getting my bill lowered when I wasn't even trying. If you're ever at the Roseville Xfinity Store, ask for Paul; he's the best and then some.
Then my wife gets home from bar bingo with the ladies, where they won $1,100 and split the winnings, so Saturday was one f*ck of a day.