Thanks, everyone. Very helpful. I was confused because some of the law practice websites have incorrect information. For example, I read tips, like the one below, which made it seem like if your green card has the code "CR1", then you do need to file the I-751. However, our green card has "CF1", which seemed not to require the I-751 (untrue). Also, I swear I read on another site which I can't find now, that even if the green card expires, it's not a problem. Anyway, we're all set now and thanks. We'll file the I-751.
From a law firm website:
"TIP:
You can check whether you have conditional residence by looking at the front of your green card. If your card has the letters “CR1” under the “Category” heading, then you have a conditional green card. You can also check the expiration date on your green card to see if you were issued a 2-year or 10-year green card."
One thing I noticed is that the USCIS appears to be allowing an additional 48 months beyond the expiration of green cards. This is for those who have filed on time the I-751:
https://www.uscis.gov/newsroom/alerts/uscis-extends-green-card-validity-for-conditional-permanent-residents-with-a-pending-form-i-751-or