I've done it and it's possible. I took a set of 69 seats and installed 68 covers on them. You will have to cut and reshape the foam a little, have lots of batting and foam on hand, and possibly add or move wires and/or crossbars, depending on what year/style seat and you are starting with and what you are installing. For example, in one set, I had a standard seat and was converting it to a deluxe one. I had to add a stiff wire horizontally across the seatback at about 3/4 height so that the area to be hog ringed would match up.
I am sure any auto interior shop can do this for you but if you are even remotely handy and have patience, you can do this yourself. If you live near south Florida, I would be happy to do these for you. I have done quite a few interiors on first gens and have always had them come out terrific.
I hoped this help and good luck.