There's a difference in the front sofas from 1972 to 1994. The 1972 sofa ia a big block of foam on the seat part and another big block of foam for the back that are covered like a giant pillowcase. It's more of a "mattress" than a sofa. The whole thing is removable from the sofa frame.
The 1992, is foam that is tufted and has separate pieces of foam on the seat, that are stitched in a manner to resemble a conventional sofa. The back is made in the same way. It has metal parts (like springs and a metal frame) as an integral part of the seat and back. It is not a pick up and take to the sewing machine kind of thing. It takes some upholstery skills to take the 1994 sofa apart and replace the foam, then upholster and tuft the new foam.
FWIW we had a sofa in our 1992 reupholstered for about $800 and the one in our 1975 done for $300. Both prices included the fabric.
Hope this helps.