2021-03-19 : If an effect such as that of Magus of the Moon causes Urborg to lose its abilities by setting it to a basic land type not in addition to its other types, it won't turn lands into Swamps, no matter in what order those effects started to apply.

2021-03-19 : Land cards not on the battlefield aren't Swamps while Urborg is on the battlefield.

2021-03-19 : Urborg's ability causes each land on the battlefield to have the land type Swamp. Any land that's a Swamp has the ability "{T}: Add {B}." Nothing else changes about those lands, including their names, other subtypes, other abilities, and whether they're legendary, basic, or snow.

2021-03-19 : Urborg, Tomb of Yawgmoth isn't a Swamp while it's not on the battlefield.