I have to change down now.

我现在得减速了。

I must change down now.

我现在必须放慢车速了。

But even may admits that may change down the road.

但是他也承认这种现状可能会发生改变。

They have persisted with little change down to the present day.

他们坚持变化不大下降到目前的一天。

I'm not an experienced driver and even have to change down on small hills.

我不是一个有经验的驾驶员。在遇到小山坡时，我甚至不得不放慢速度。