Readings & Meanings:
I switched in a new tatami mat.
I'm going to exchange yen for dollars.
I exchanged yen into dollars.
Here's an oddball radical: 曰, or Hirabi.
I know it looks like 日, but Hirabi's middle line doesn't quite reach the right side. 曰 means "say" and is a picture of a mouth (口) with a suggestion of a tongue in the middle. In 替, though, it's just an action indicator.
(In fact, 曰 was replaced by 日 later on, for simplification. I say, too little, too late.)
On top we've got a doubled husband: 夫. 夫 is the standing figure of any person (doesn't have to be husband).
This kanji then shows one person taking the place of another.