Let’s Encrypt to Transition to ISRG Root

By issuing from the cross-signed X3 intermediate, the client will recognise the IdenTrust root CA as trusted and thus recognise the intermediate and leaf certificate as trusted. On July 8th, 2019 your new Let’s Encrypt certificate will be issued from the X3 intermediate that chains to the ISRG root, but your site/service might continue to serve the cross-signed X3 intermediate during the TLS handshake. If you get a new certificate issued from the X3 intermediate that chains to the ISRG root, but you serve the old, cross-signed X3 intermediate in the TLS handshake, the connection will still work (* for now, keep reading).

Source: scotthelme.co.uk