Is upgrade to fiber internet mandatory?

As far as I know, it is not [yet] required.  You have been called by a pushy sales person who is stretching the truth.  If you want to upgrade, you might want to use the website instead.

AT&T would like to shut down as much of the copper networks that they have as possible. Some states require the copper phone system to be left intact. There is no requirement to upgrade to fiber. It may be worthwhile to upgrade just to get newer equipment and more reliable service.

As the previous replies have said, don't believe it's required.

Assuming you have DSL. Think of DSL as an old beater of a car that's served you well, but it's owner, AT&T, isn't exactly sticking to a strict oil change schedule or worrying about balding tires on it. And they put gas into it a gallon at a time. It's going to break down over time more frequently.

And so if your internet connection is important to you (maybe you need it for work?) and it doesn't cost you any more than you're paying now (I haven't seen dsl pricing in a long time -- but I hear people complaining about the diminished value of it) it probably makes sense to upgrade on your schedule rather than when an outage forces you to.

Thanks, everyone.  I have nothing against switching to fiber, only that the salesperson was deceptive in implying that the obsolescence of my current equipment was imminent and that I had to upgrade NOW.  I'll eventually switch, but at the moment everything is working fine and I'd prefer to do it in my own time rather than being tricked into doing it for some salesperson's benefit.

I did, by the way, confirm with AT&T that upgrading is optional at this time.  If anyone else experiences such deceptive sales practices and would like to report it, the number I called was 855-920-0146.

Another benefit might be a promotion. I don't think I'm paying that much more (if anything) for fiber 1000 than I was for copper at 50Mbps. It's even possible I'm paying less.

Of course the 1000 upload speeds are also nice.

If you upload a lot of stuff and/or do a lot of video streaming, it may be worth looking into... especially if there's a good price involved.

I'm currently signed up for a 40 Mbps service from ATT. The terms  do not specify copper or fiber. I'm connected via coax.  For the past month the performance has been very poor (usually < 20 Mbps with frequent outages, necessitating equipment resets). I don't understand why I need to sign up for a new service, just to get the promised performance of my existing service. In spite of ATT's "...at no additional cost" claims, I expect that the new service would end up costing me more. If ATT can't deliver on my existing service, they should fix it. If that involves switching to fiber, that's OK with me.

You might be connected via coax from the NID to your aging Gateway with a coax connector.  Modern Gateways do not have the coax connector at all.  An upgrade to fiber cannot include that coax leg.  AT&T's copper plant is degrading with time.  It may be possible to correct the issues that are causing you problems, assuming you call them and ask for resolution to your issues.  (If you'd like, you can open a post about your issues and provide a screenshot from your Gateway's Broadband tab for analysis... as this is a necro'd post on a slightly different subject).

You are correct to be concerned about the "No additional cost," because the sales people tend to forget than when entering your order and put you in a more expensive package... so your vigilance after the install to make sure you get what you were promised may be necessary.  Ideally use written materials or the website to order so you have the offer in print.

Fiber will typically give you better latency as well as better bandwidth.  And it's new plant, and typically more reliable than the aging copper.

@fhammer 

ATT hardwired services do not have internet 40.

FTTP is 5000, 2000, 1000, 500, 300

FTTN is 100, 75, 50, 25

ADSL+2 is 18, 10, 5, 3, 1.5, .76

There is a G.FAST internet in some buildings using coax or Ethernet but I believe those speeds are faster 40. 
Likely starting at 150 and going up.

@my thoughts ,

ATT hardwired services do not have internet 40.

I have found that most people have no clue what they've actually signed up for.  But that's a good point.  I wonder if OP has 32, or 50?

It depends on multiple factors, let's say they are upgrading their infrastructure then you might need to requip with the latest technology.