On June 12, 2018, USCIS announced that as of June 11, removal of conditions applicants can use their receipt notice along with the old green card as evidence of continued status for up to 18 months past the expiration date. This was formerly 12 months. The fact they're extending it from 12 to 18 months is further evidence that everything is taking longer in immigration, across the board. We should all expect delays and timeline extensions continue for the foreseeable future.
Here's the USCIS alert word for word:
"As of June 11, 2018, petitioners who file Form I-751, Petition to Remove Conditions on Residence, or Form I-829, Petition by Entrepreneur to Remove Conditions on Permanent Resident Status, will receive a Form I-797 receipt notice that can be presented with their Form I-551, Permanent Resident Card, as evidence of continued status for 18 months past the expiration date on their Permanent Resident Card.
We are making the change from 12 to 18 months because current processing times for Form I-751 and Form I-829 have increased over the past year.
Additionally, we will issue new Form I-797 receipt notices to eligible conditional permanent residents whose Form I-751 or I-829 was still pending as of June 11, 2018. Those Form I-797 receipt notices will also serve as evidence of continued status for 18 months past the expiration date on petitioner’s Permanent Resident Card.
As a reminder, conditional permanent residents who plan to be outside of the United States for a year or more should apply for a reentry permit by filing Form I-131, Application for Travel Document, before leaving the country."