I'm trying to increase the slope of my substrate by adding new eco complete on top of my existing eco complete. Planning to add about 15 pounds to the existing 100 pounds already in the tank, which has been set up for over a year. I read some posts saying that this would start a new cycle and endanger the fish. Is this true? This doesn't quite make sense to me since eco complete is inert, my existing substrate is still there, and the filter media is untouched. Thoughts?

The bacterias that process ammonia and nitrite are aerobic, so when you bury them, they die. And since you only have as many bacteria as are necessary to process the available ammonia and nitrite (it's not like you have any extras just hanging around - when they don't have enough food, they die), when bacteria are killed off, there are suddenly not enough to get rid of all the natural byproduct of decaying organic matter. However, the change is often insignificant, plants can take up some slack as they will use ammonia as a nutrient, and bacteria reproduce quickly, so you might not notice.

But, you might get a mini-cycle. It depends on things like how much filter media (including prefilters) and random equipment and hardscape you have in the tank, as well as total plant mass. The more total surface area there is, the less bacteria there are per given two-dimensional area. Just test daily for ammonia for a week or so, and if you see any, do a big water change.

+1 for kevmo911
I'm sure someone else may have additional info, but Neon Tetra's are extremely sensitive to Ammonia. I would remove any from that tank first.
Since bacteria multiply quite rapidly, and you are covering up only 5-10% of them it
should be "back-in-business" in 24-48 hrs.

Makes sense. How deep do the aerobic bacteria live? If I scoop up the top 1/2 inch of my 3 inch existing substrate, and then pour on TOP of the new substrate, would that help? Or would that just release more ammonia from disturbing the substrate

Yes it would help keep the your cycle bacteria alive; but you will be into another mess by disturbing the substrate you might dislodge some organic matters entrenched there which might lead to a bacterial/algae bloom and even worse an ammonia spike.

Better stick to your original plan - that is less of a risk - the plants prefer ammonia to any other form of nitrogen. Just do a few quick water changes after adding substrate and I think you will be all right.