When a land is granted a basic land type, what effects does that have?

Question

For example:

I imagine this card is useful specifically because it denies the opponent the appropriate color of mana, if they are not already playing a blue deck. If I make a Swamp into an Island, then it produces U instead of B.

But what about with non-basic lands? If I turn Hinterland Harbor into an Island, does that mean it can no longer produce green mana? What about Seat of the Synod? Will that still be an artifact?

Answer 1

Keep in mind that there is a difference between "gaining" a basic land type (Aquitect's Will, "in addition to") and a land "becoming" or "is" a basic land type. When a land becomes a basic land type, it loses its old rules text and gains the appropriate implict basic land mana generating ability. Supertypes and Types are unaffected, so a land that is also a creature or artifact remains one.

305.7. If an effect changes a land's subtype to one or more of the basic land types, the land no longer has its old land type. It loses all abilities generated from its rules text and its old land types, and it gains the appropriate mana ability for each new basic land type. Note that this doesn't remove any abilities that were granted to the land by other effects. Changing a land's subtype doesn't add or remove any card types (such as creature) or supertypes (such as basic, legendary, and snow) the land may have. If a land gains one or more land types in addition to its own, i keeps its land types and rules text, and it gains the new land types and mana abilities.

Answer 2

The land becomes an Island. It can no longer produce anything except U mana without help from other cards.

It would have the text 'in addition to its other types' or something similar if it meant anything else.