I think the announcement you're looking for is this one:
As of May 4, the default chain will go from Leaf ← R3 ← DST Root CA X3 to Leaf ← R3 ← ISRG Root X1 ← DST Root CA X3, but it's the same root in both cases. (The alternate chain is not changing, and will remain Leaf ← R3 ← ISRG Root X1.) What you need to worry about is actually in September, when DST Root CA X3 expires and old version of OpenSSL won't trust any chain including it (even if the chain includes ISRG Root X1 which it would have trusted in and of itself).
So, if your systems already trust ISRG Root X1, you want the "alternate" chain that doesn't include the expiring DST Root CA X3 in it. But in fact "ISRG Root X1" will be in both chains starting May 4, rather than only being in the alternate. So it really depends on how your client selects the chain how things will work then. The right string is almost definitely "ISRG Root X1" to select that root, it's just I don't know if your client will be confused if it sees that certificate in the default chain and think that's what should be selected instead.