I changed all mine.
It is worth using a bin liner between the foam and the new covers, it really helps when sliding them on, small cable ties are the way to go, the metal rings need special pliers to form them, I think they're called hogrings.
Also take your time when you remove the plastic trim around the base of the seats, it's quite fragile.
Something else I've just remembered, the plastic trim on the seat back is a nightmare to get off, ideally you need one of those trim removal tools that are like a thin flat bar with a handle