How can I make a futon into a sofa?

You can add wood to the legs and finish it to match the rest of the futon frame. One way would be to look at the finials you can put on the top of rails on stairways, put them on the floor and add wood to make it the height you want above that..

a picture might help give ideas

Can you replace the legs?

They make leg risers for beds, sold at bed bath and beyond. May be they would fit

You can buy wooden furniture legs at some stores. Just do a google search in your area. Amazon sell them too - https://www.amazon.com/Wooden-Sofa-Legs-Turned-Feet/dp/B00CJ38L7K. Maybe it will help raise your futon

When working for the disabled and folks with hip or knee injuries, we have to increase the height of sofas/chairs buy either replacing the legs with new taller legs, or I have had people add layers of two by fours to the bottom of sofas along the front and back edge to increase height. You can also just add a platform to the bottom of the futon, using 4x4s and then use a sheet of plywood on top, then screw your futon on top. I've used large cinder blocks as a quick fix for myself after a knee injury, just put the futon legs in the holes of the cinderblocks to keep it from moving.