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In CR 305.7:

If an effect sets 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, its old land types, and any copy effects affecting that land, 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. Setting 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, it keeps its land types and rules text, and it gains the new land types and mana abilities.

is the phrase "It loses all abilities generated from its rules text" unique for land changers like Blood Moon or Spreading Seas, or is it derived from another rule?

In other words: is there a rule about setting types, from which we could derive, that if a "All creatures are elves" card existed, then it would turn all creatures into vanilla elves?

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That rule you quoted right there is the one that makes lands lose all other abilities in that situation. There is no more general rule that applies to subtypes of other types, so your not-so-hypothetical "All creatures are [specific creature type]" would not remove any abilities (see Conspiracy). You wouldn't have to search the whole Comprehensive Rulebook to be sure that such a rule does not exist. The rule you quote is in the Type section, Land subsection of the rules. Logically, if such a general rule existed, it would be in the General subsection of the Type section. That list is pretty short, and it's clear that that rule is not there.

In fact, it wouldn't even make sense for that rule to be derived from a more general rule like the one you propose. The rule you quote specifically applies to setting a land's subtype to a basic land type (Plains, Island, Swamp, Mountain, Forest), so setting a land's subtype to another land type like "Gate" or "Locus" or "Urza's" doesn't do anything to the abilities.

More broadly, rules in the Comprehensive Rulebook are generally not "derived" from other rules. You can find some duplication in there, with specific rules listed under multiple numbers, but other than that each rule is there for a reason and does something that none of the others do.

Functionally, the rule you quote acts as a dual to rule 305.6:

The basic land types are Plains, Island, Swamp, Mountain, and Forest. If an object uses the words “basic land type,” it’s referring to one of these subtypes. A land with a basic land type has the intrinsic ability “{T}: Add [mana symbol],” even if the text box doesn’t actually contain that text or the object has no text box. For Plains, [mana symbol] is {W}; for Islands, {U}; for Swamps, {B}; for Mountains, {R}; and for Forests, {G}. See rule 107.4a. See also rule 605, “Mana Abilities.”

With rule 305.6, adding the type "Mountain" to a land makes it act like the card Mountain in addition to whatever else it does. With rule 305.7, setting a land's subtype to "Mountain" makes it act like only the card Mountain.

  • Re "rules in the Comprehensive Rulebook are generally not "derived" from other rules.", This is true, but the number of derived rules is still quite large. I call them "reminder rules". – ikegami Jun 3 at 12:18
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    Can you point me at some examples? – murgatroid99 Jun 3 at 15:25
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Such cards add creature types instead of setting them, e.g. Arcane Adaptation:

As Arcane Adaptation enters the battlefield, choose a creature type.

Creatures you control are the chosen type in addition to their other types. The same is true for creature spells you control and creature cards you own that aren't on the battlefield.

(emphasis mine)

A card that sets a creature's type is Omnibian ("Tap: Target creature becomes a Frog with base power and toughness 3/3 until end of turn."); note that the rulings below the card in Gatherer explicitly mention the fact that the target does not lose any abilities. A card like Amoeboid Changeling has the ability to make a target creature lose all creature types, without losing any abilities. The connection between a subtype and abilities is simply much stronger for lands than for creatures, so therefore there is no equivalent of rule 305.7 for creatures.

For things like equipments, the connection is stronger, and 301.5c says:

An Equipment that loses the subtype “Equipment” can’t equip a creature.

I think that's the closest and only equivalent of 305.7 (by 301.6 it also applies to fortifications w.r.t. lands); note that there's no similar rule for Auras.

  • The precedent for setting creatures types without adding them is the card Conspiracy, as I mentioned in my answer. Amoeboid Changeling isn't super relevant here, because if a land loses all of its subtypes it does not in general lose its abilities either. – murgatroid99 Jun 3 at 7:28
  • There's also stuff like [mtg: Omnibian] – Acccumulation Jun 3 at 15:08
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The rule quoted is an exception. Lands specifically have the rule that if they become a type then they lose their text, unless they become a type "in addition to its/their other types". This is not true for any other card type. So no, if a creature "became an Elf", even if not "in addition to its/their other types", then it would not lose its text.

301.5c states that Equipment that loses the type Equipment can't be attached to a creature (and 301.6 for Fortifications). The text of the equipment does not change; it still has the Equip (or Fortify) ability, and you can still pay the cost and choose a target as normal; the ability will simply fail to do anything on resolution.

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