As an AT&T U-Verse customer, what functionality will I lose is I drop AT&T internet?

I’m pretty sure uverse only works with uverse internet. It’s been so long since I’ve dealt with that side. Of the house. Hopefully someone that knows more than me at this point will chime in 

I don't believe a customer needs AT&T Internet to use U-Verse.

There was a time a long time ago when a customer needed matching internet+TV to access apps to watch TV on, but I don't think this is the case anymore.

A customer will probably lose bundle pricing.

Comcrap is the worst though.

You don’t need ATT internet service but I’m pretty sure your UVerse receiver must still use the ATT connection, it can not use the XFinity connection. You will basically still have ATT internet for your UVerse receiver only. If you want ISP independent TV service from ATT, that’s ATT TV not UVerse.

True, but it depends on how the OP interprets Uverse TV.  He’ll still need the gateway to allow the DVR and settop boxes to function.  His bill would only show the TV service, so that should be okay.   

What @gr8sho said. The customer would still need the AT&T gateway, but the internet portion wouldn't be active.

So, if I understand it correctly, so long as I keep the gateway (I assumed that would be the case), I would still have full access to TV, DVR, On Demand, and all the rest.  The only difference being that the other devices in the house would connect to a different modem/router.  Presumably, I'd lose the ability to manage the AT&T Gateway through my desktop computer, since it would no longer appear on the LAN (not that that's a deal breaker).

Yes. You must keep the gateway. I believe you can still manage the gateway via the 192.168 address.

Oh and if you have Internet 1000 with the complimentary HBO Max, you'll lose your account and have to start from scratch.

I currently have 50 mbps service via two twisted pair, which AT&T tells me is as good as it's likely to get in my location (pretty impressive, actually, but that's another discussion).  HBO Max is not part of the deal.  Thus, my desire to upgrade to 400 mbps from Xfinity for the same price. 

Another thought - I currently have a Cat-6 line running from the gateway to an 8 port switch in the ManCave which feeds the U-Verse receiver, the TV, the DVR, and a few other devices.  I assume I'll need to run a separate cable for this?  Not a big deal, but something to consider.

Why do people keep thinking Xfinity is a company? *Facepalm*

"Comcast" doesn't appear anywhere on the latest promotional mailing I received.

Comcast is still the company that sells a product known as Xfinity. Basically, Comcast had so much negative feedback that they "rebranded" their TV and internet services as Xfinity in hopes that customers would be bamboozled into thinking it was a new company

https://en.wikipedia.org/wiki/Xfinity