No. The 980 has different engines (yes, they are all -10s, but the conversions are -10Ts and the 980/1000 are -10-511k). The 980 has 733 horsepower on takeoff, versus 717, and it has single red line computers for engine management.
So there is a difference. I believe there have been a few 840s truly converted to 980s, but that conversion process was much more expensive than just puting -10T engines on, and arguably you didn't get much more.
The easy way to tell a 980 from at 840 by looking at the panel is that the 980 has two "Auto Start Override" buttons somewhere on the panel, usually in a prominent spot. They are large buttons with lights in them, and pretty easy to spot.