You can shorten the length of your mortgage with the new bank when you switch, subject to the usual qualifying criteria ie affordability & LTI/LTV. But once you shorten it, it might be tricky to increase it should you have too if something happens like job loss, unexpected bills like medical, nursing home, education. Safest method is to keep your original term, and structure your mortgage in the way that allows you to repay early, in line with the 20 years. This could be part variable, fully variable, or fixed with a bank like BOI, UB or KBC that allow varying degrees of overpayment on a fixed rate without penalty.