These two parts have interchangeability.

These two software are completely interchangeable.

He fought a court battle to gain custody of his child.

She undertook the custody and care of the child.

This new software has backward compatibility, so you can use old version data without any problems.

He was thinking of a catchphrase to come up with something he wanted to say.

They were having fun chatting at the party.

I stayed at home to watch over my sick grandmother.

She is in charge of watching over the patients at the hospital.

I had a good time having fun talking with my friends.