Unable to upgrade phone without an unlimited plan?

Nope, not true.  I suspect this was just some tactic to try to get you to change plan so that they could take credit for it.

If you are trying to take advantage of the upgrade with trade-in offer, you cannot do that and keep your shared plan. The upgrade with trade-in offer requires that you switch to an unlimited data plan

I have personally done the upgrade at Best Buy for myself, with an old shared plan, without a problem.   If you want to keep your old mobile share plan, you need to let the person who is activating your phone know to contact AT & T and make sure they do not take you off your old plan.

One recurring problem is combined billing. You cannot upgrade outside of AT&T if you have combined billing without De- combining first. 

Post script.... 

All carriers are just unlimited now.  The alternative is a very low data plan with overage charges that cost as much as unlimited data, or prepaid. Even AT&T's current limited data plan is ridiculously expensive and charges overage

You're actually right. The AT&T CSR was outright lying. I was able to do the upgrade at walmart.com with no issues. The problem is probably specific to Best Buy not able to connect  to AT&T system to confirm the installment agreement, and instead of sending me to someone more technical, the AT&T CSR wanted to lure me to one of their overpriced unlimited plans. 

The shared plan I have now is far cheaper for 3 lines ($115 with tax and all), has 9GB shared data with rollover, plus if I exceed my data limit, there is no overage charge, it just slows down to 2G speeds. I don't recall I ever used up all my data. God bless WiFi. 

We are happy to hear that you were able to upgrade your phone with your retired plan at Walmart hmoubarek!

Most of our device upgrades/trade-in do require a current AT&T Unlimited plan, retired unlimited plans may also qualify (excluding prepaid, session-based, and Data Unlimited at 45/mo. plans). We'll be happy to step in and submit any feedback for areas of opportunity. 

We hope this helps! Reach back out to us if you have additional questions. Thank you for reaching out to the AT&T Community Forums!

Rhoda, AT&T Community Specialist 

I understand that most $800 off a new phone with trade-in deals require an unlimited plan. What I was trying to do here is the regular device upgrade with installment plan, which is available to all lines regardless of the data plan. 

What is really frustrating here is that instead of looking into the error I was receiving, or escalating the issue to someone more technical, the AT&T phone rep made up a new rule that my line is not eligible for a device upgrade.

This tactic of pushing down unlimited plan down customers throats before looking into any issue is actually making you lose business. I was actually determined to switch carriers if I weren't able to upgrade with another store. 

Hi hmoubarek! Let's address your upgrade inquiry with a grandfathered plan!

We hear you and we do appreciate the feedback. We'll go ahead and submit the feedback to the relevant team for areas of growth and opportunity. 

Thank you for reaching out to the AT&T Community Forums!

Rhoda, AT&T Community Specialist 

This post/thread has been really helpful. I went to trade in my phone and upgrade yesterday. Everything went great. The sales associate, however, did not once tell me my plan was also changing to unlimited data. My bill has doubled 😖 and I don’t need unlimited data. I was perfectly comfortable with my 10GB plan. Is there any way to change back to my old plan? Am I just stuck with this??

Return the phone.  And pay full price for the phone.  All phone deals require a unlimited plan.

@JDoe If your plan was changed without your knowledge or consent, you should be able to get the old plan back but it might take filing an FCC or BBB complaint. However, if you did a trade in deal that gets you a discount on the new phone, that deal requires having an unlimited data plan. Also, your bill has not doubled. If you just did the upgrade yesterday, you haven’t gotten a bill yet. What you got was the customer service summary also known as the projected bill and they are infamously wrong because they don’t include any discounts or phone credits. 

@JDoe  look at the website and do the math, with and without installments. (remember the promotion doesn't credit your installments for the first two or three months)

I was paying for a 10 gig plan with a 22% discount the five lines and a tablet. $168

If I add phone installments on top of that $168, my bill increases by at least $125.  Makes 5 lines on starter @$30 each very reasonable.