Transform turns a double-faced permanent over to its other face. The card stays on the battlefield — it doesn't leave and re-enter — but it now uses the name, card types, text and power and toughness printed on the back face. The instruction often comes from an activated ability, as in “{5}{B/P}: Transform Rona. Activate only as a sorcery,” so you pay the cost and the change happens at sorcery speed. That reminder text also notes that a {B/P} symbol can be paid with either one black mana or 2 life.
Other transform cards flip when a triggered condition is met instead. Either way, one card plays like two: the front face sets up, the back face cashes in, and any counters or Auras on the permanent survive the change. The natural answer is to remove the permanent before the transform resolves — if it leaves the battlefield, nothing flips.
