The old leaves had enough light so they were concave. The new leaves that grew in suboptimal conditions are concave because they are doing all they can to accept as much of the inadequate light as possible.

Concave = fold inwards because its too bright
Convex = open up, there is not enough light

Oh wow. I suspected it had something to do with light, but I totally missed that the convex shape itself opens up more of the surface of the leaf trying to catch more sun rays. Incredible how plants do that. Thank you for the reply.

As it were, I am researching grow lights for my apartment right these days, so maybe that will make this ficus happier.

I would not recommend grow lights for an ornamental plant like this where the main purpose is for the plant to adorn your apartment. This is a low light plant and convex leaves are not an issue. The plant is simply adapting to its environment and this plant is extremely efficient on this score.

I have three ficus elastica burgundies in my apartment. Two of which are in low light areas. One of those two is lucky to get 500 lux and is still growing new leaves.

If you find yourself insatiably possessed with adding light then I might recommend just replacing a nearby light bulb with a stronger 'daylight' (5000-6400k) CFL bulb. That's all you'll ever need with this plant.

Hmm. Well, a major reason I do not have grow lights yet is actually because I think they look totally ugly and take away from enjoying the space by humans. Definitely something to think about.

I have other plants as well - most not high light but I am very curious how they'll do with much better lighting.