LOL. I meant mass conversion within the North America.
I can't recall any. A diocese of Nestorians did a few years ago.
My parish was considering going to the Vatican when they were going from Bible study to apostolic succession, and their leader, a former communicant of the Vatican IIRC (it was long before my time) pushed it hard. The archdiocese, however, would not receive them, and would have broken the parish up and distribute them among declining parishes. The priest shortage was also an issue, and they wouldn't ordain the pastor, who left with some of the members and went to the Vatican anyways.
The parish (or at that time, congregation) had a friendly association with a congregation of Pentacostals (IIRC) who did go to the Vatican and founded "This Rock" magazine. They were rather disappointed that our congregation went Orthodox.
I vaguely recall some Lutheran congregations jumping in the Tiber.
It happens, but most of the time they just get absorbed into an already established parish-we have no shortage of them here, and most could use a boost in numbers and enthusiasm.