Do I have to upgrade my plan if I buy a 5G capable phone?

My current plan does not include 5G. 

Yes it does as all plans do. The generation of cellular signal (3G, 4G, 5G, etc.) does not care what plan you have. What matters if you phone is built for 5G or not (think of it as a 5G antenna).

As long as your phone is on the list of AT&T supported Voice over LTE (VoLTE) phones you will be fine. Do not get a phone that is LTE only for data with 3G for voice.

Yes it does as all plans do.

@Juniper No, that is not true. There are many grandfathered plans that do not include 5G access. Even one of the current plans, the 4GB plan, does not include 5G.

@sandblaster 

Thank you for the correction. I have never had the experience of needing a new plan just to take advantage of the generation of service, just as long as I had a compatible phone. So though I've had grandfathered plans, never one that restricted my phone's antenna like those you mention.

Yes, it is a grandfathered plan with 30GB of data monthly.  I'm concerned that if I try to bring a new 5G phone onto the plan, they will not activate the data plan for it and force me to migrate to a new 5G plan.

@rwproctor No need to worry, they won’t do that. You can keep your plan.

So it will work, just not for 5G itself because of the limitation of the plan. Not a model of plan I agree with, but that's a grandfathered plan for you.

any recent update about this? Was told today I needed to update my plan.

@turtlejacki56 Why were you told that? What plan do you have? There is still no requirement to upgrade to a 5G plan because of buying a 5G phone. There could be other reasons you need to upgrade your plan such as to qualify for a trade in promotion.