It would help if one could see what they are like. Can you unzip them and see what is in them now? If it's sunk to the bottom it is probably foam chips that have disntegrated.

It really depends on the look you are going for. If they are square cushions I would go for 4 or 5 inch solid foam. Memory foam is best with a single or double wrap of quilt batting. If you want them poofy, go for a narrower memory foam base and wrap in quilt batting with smaller pieces of batting inserted until you get the shape you want.

I had down stuffed back pillows on one couch, and I did not like how the feathers all sunk to the bottom even while sitting on the couch. I felt the wider pillow bottom pushed me out of the couch and the pillows had to be fluffed constantly. I had the down removed and solid foam put in the pillows, much to the consternation of the man who sold it to me.

I did this years ago when I didn't have much $ I was given a 2 year old, custom made sofa . It had down back cushions. I hated them. I went to JCPS and purchased pillow protectors, with the zipper. I filled them with shredded foam as it was the cheapest for me at the time and wrapped it in solid foam. They were hard to slide in so I bought cheap satin pillow cases to cover the foam and they just slid right in. If you plan on using solid foam and have to cut to fit use an electric knife, with little pressure and let the knife do the work. You will get a straight edge this way. This also works to cut Styrafoam for a clean edge.

You can also use plastic garbage bags to help slide foam into a cushion. Split the bag down 1 side, lay the foam inside it, slide the plastic covered foam into the cushion. Then just pull on the side you didn't cut and the bag slides right out. Might take a bit of manuvering the first time you try, but easy to get the hang of!