Land type changing effects that change a dual land's land type will remove the old land types completely. Text-changing effects that just change one of the two land types will leave the other type unaffected.

This card is a Mountain and a Swamp even while in the graveyard, library, or any other zone.

This has the mana abilities associated with both of its basic land types.

This has basic land types, but it isn't a basic land. Things that affect basic lands don't affect it. Things that affect basic land types do.