We keep replacing the old ones who die off.

"But now we love them very much, so I just keep replacing their parts when they become damaged."

He tried to take off the oxygen mask but the doctors kept replacing it.

But in the process, I kept replacing the older songs, one song at a time.

Of course, they had to keep replacing this huge population.

He doesn't, but he's so used to having one, he keeps replacing it.

The only problem is, I have to keep replacing the heads.

They held for a while and he had to keep replacing them.

The air keeps replacing itself, more or less, by volcanic action.

As he spoke, sets of musicians kept replacing one another.