You are supposed to replace it with a card that expires, but, frankly, you should avoid renewing that card for as long as possible as it saves you hundreds, over the long run thousands of dollars!
If you travel internationally with the card and return to the US, it's *possible* that the CBP officer informs you that you need to replace the card with another one. You'll then say "I'll do that right away when I get home" and then you of course don't do it. The card is still valid, and that aside, you are a LPR regardless. There's no punishment or late fee or any of that nonsense for an expired card.
Congratulations on having one of those!
An immigrant from Europe, I live on the American Riviera and work as an attorney in Santa Barbara, California.