rebore the whole cylinder 0.5mm and not changing pistons? Then your piston-to-wall clearance is gonna be shit, possible issues might include rattling noise when cold, or even when hot, excessive wear and tear, loss of compression and oil loss.
Usually if rebore 0.5mm it will include changing the pistons to 0.5mm oversized as well, i have never heard of anybody boring 0.5mm oversized and not changing pistons. And reboring 0.5mm is considered oversized already what so what you mean by not going oversized?