Excerpt:
LONDON MULLIGAN TESTING
From April 10 to May 1, the London Mulligan will be the mulligan used on Magic Online. This mulligan is part of an experiment run by Magic R&D in their never-ending efforts to improve the game we all know and love. This will apply to all formats, in all events and casual games.
As MTGO is the best and largest way to play classic formats like Legacy and Vintage, this test will provide a critical mass of data to help R&D make a final determination whether to adopt this mulligan for Magic as a whole. And importantly, it will allow Mythic Championship competitors to practice their Modern game under the same conditions that they'll face in London.