I just replaced cheap soft cushions with new cushions that are now too firm. Without replacing again - is there a way to soften them up? Maybe cutting in half and adding softer foam? Please help.

Hello, I'm a low level or "newbie" DIY-er. Over the years I've repaired cheap furniture (fixed a few IKEA couch frames), reshaped and reinforced some warped MDF furniture, etc but have never done anything complicated. Anyhow, I recently purchased an Italian leather sofa via Craigslist that is nearly unused and beautiful... but somewhat uncomfortable! It's mid-century inspired and as such the seat and the back are not as ergonomic as more modern designs. They're pretty straight. After about 15 minutes sitting upright, you get some discomfort in the lower back and need a pillow or a folded throw blanket to relieve the tension. I'd like to fix this at the source! My idea is that I could either tilt the back cushions a bit more using some sort of hidden "shim" so they hit you more in the lower back, or actually open up the cushions themselves and insert some additional shaped foam to round out the lower-back area. The way this sofa is designed, none of the cushions are removable; they are sewn to the canvas that covers the frame beneath the cushions. I could easily get to the seams with a seam ripper or blade, but I cannot fathom how I could re-attach them without doing it by hand, which would be cumbersome and not nearly as sturdy. So this is a two fold question: a) Does anyone have any suggestions on how to bolster/improve the lower back support of a fixed-cushion couch, internally? b) Any advice on how I could re-sew the cushions once the frame is modified? (How on Earth did a machine get into those crevaces in the first place?!) Thanks all!