The mirror of a fallen-angel. Crossing into investment-grade adds the issuer to indices tracked by trillions of dollars and makes it eligible for insurance portfolios, so demand expands far more than supply.
Most of the spread tightening happens in anticipation, when the agencies move to positive outlook rather than on the upgrade day itself. Trading the upgrade after it is announced is usually too late.
Example: an issuer's 10-year bond trades at a 290 basis point spread as a BB plus credit. Both agencies move to positive outlook and the spread grinds to 180 over four months, a 110 basis point gain on a bond with spread-duration near 7, worth roughly 7.7 points of price.
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