I kinda meant, since the new astra is based on the same platform as the Cruze, then they should presumably bolt right in to the Cruze. Will they also fit the sonic with little to no modification?
Yes, they should. Seats are typically only 4 bolts and maybe a few wiring harnesses for airbag sensors and what not. I wouldn't expect it to swap right in and use the same harness and bolt holes but with some ingenuity it should work.