If I get a 5G phone, such as the iPhone 12, will I have to upgrade my plan, or can I still use 4G on the new phone?

No, you don’t have to upgrade your plan if you get a 5G phone. Obviously, you won’t get 5G until you do but ATT will not force the change, at least not yet. Eventually, I’m sure they will.

Thank you very much!

That response is incorrect...I just inquired tonight with AT&T as to if I upgraded to the new iPhone 12 (or 12 Pro), would I be forced to change my existing 4G plan to an Unlimited 5G plan??...and they told me that “Yes, you are required to change your plan & the only plan compatible, is their Unlimited Elite plan (which no surprise, is their top tier & most expensive plan)”.  So they are trying to force subscribers to change to the most expensive option, if they wish to use the new iPhone 12.  This is absolutely ridiculous...I told them that, if that was the case...I’d take all 7 lines & move to a different carrier!!!

@MichiganFan Whoever told you that is wrong. Not only are you not forced to change to a 5G plan, all 3 of their current unlimited plans are 5G. Unlimited Elite is not the only one. I realize you won’t take my word since I’m not an employee but there is another thread with the same question with a response from ATTHelp that confirms a plan change is not required. 

Edit: Here is that other thread I referenced: https://forums.att.com/conversations/samsung/do-outside-purchased-5g-capable-phones-require-a-5g-plan/5f7e9e1a72a09d33d0aed3dc?commentId=5f8617ddc17a065683f92e68

what does it mean when my phone says 'Mobile data turned off'?  When it turn on I can not connect to the my internet.

In AT&T store, sales rep strategy is to get you to trade in your old phone on the iPhone 12, thus triggering your Plan upgrade to one of their 5G plans, and swoon you with a discounted new iPhone 12.  In my case, my iPhone 7 Plus will only get me $85 if I trade in at AT&T, but the minimal 5G Plan is $30 more per month that my lil’ ol’ 12gig Shared data plan that I share with my Mom.  So I walked. Got home and called AT&T and the rep on the phone said because I barely use my cellular data now and nearly 10 of my 12 gig gets rolled into the next month AND my region didn’t have 5G coverage that would impress me, I should keep my iPhone 7 Plus and sell privately or just keep as an additional SONOS Remote Control.  I can then buy an iPhone 12 of any type, pay in full and keep my old data plan, 4G LTE.  The rep on the phone documented my account notes in case I encounter the switch-a-roo attempt when I buy my iPhone 12 Pro Max.     I’ve been seeing that iPhone 12 sales are kind of a dud (but 12 Pro Max comes out 11/6) so I’m wondering if it’s because of sales pressure to trick people into 5G plans that has given excited potential upgrades the cold feet.    If you live in a major metro area and spent time outside away from wifi AND stream lots of “content” to your current phone, then knock yourself out!  One thing I can’t figure out is what data needs to get to your phone faster than it already is if you already stream this content?    

5g is not about speed even tho the carriers make it seem that way.  It's about less congestion as bands are wider and agrigation is better than LTE.  Anything over 10mbps to a phone is over kill.

I was looking at the new plans and it seems the WatchTV service is no longer included in those.  To me it is a deal breaker because from what I understand, the new plan will be more expensive and I’ll have to pay extra for streaming TV channels.  Can anyone confirm this?

yeah watch TV is dead except for those old plans but ATT is going to shut it down eventually as they are no longer selling watchtv.

I had the premium plan that included watchtv and it costs me less with the new plans.

Losing WatchTv is deal breaker for me right now also. You can still get new phone for it's other features and there is a lot  same with Samsung Ultra family.

 Well it they keep rising the price of Grandfathered Phone plans Like they did before in AT&T plan changes.