When leveling in Dungeon World, if you choose a move that has a "Replaces: X", do you need to have the X move?
For example, a thief wants to take the move "Dirty Fighter". It states "Replaces: Cheap Shot". Does the thief need to have taken "Cheap Shot", or can he just choose "Dirty Fighter"?
Moves that Replace other moves do require you to have the prior move before learning the replacing move. This is explained in the rulebook in the Playing the Game section:
Requires and Replaces
Some moves that you gain at higher levels depend on other moves. If another move is listed along with the word Requires or Replaces you can only gain the new move if you have the listed move.
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If you don't have the Dungeon World book, you can find these rules in several online sources under the CC-BY and OGL for example here.
Obtaining upgraded moves is a common feature in many PbtA games, since Apocalypse World made upgrading the game's basic moves an option you could pick for leveling high enough. Dungeon World's system is one of the most complicated for doing so, likely a callback to the existence of feat trees and upgrading class abilities in D&D.
There are two fundamental mechanics for upgrading moves, both of which are worth understanding not just in playing the game but in writing your own classes for future use.
Some moves that you gain at higher levels depend on other moves. If another move is listed along with the word Requires or Replaces you can only gain the new move if you have the listed move.
A move that requires another move can only be taken if you have the move it requires already. You then have both moves and they both apply.
A move that replaces another move can only be taken if you have the move it replaces already. You lose access to the replaced move and just have the new one. The new move will usually include all the benefits of the replaced one: maybe you replace a move that gives you 1 armor with one that gives you 2 armor instead.
-- Dungeon World github, "Playing the Game", emphasis mine.
So, all of the Thief's replaces moves need the Thief to have taken the move they're replacing first.
To illustrate the difference between the two - well, the Thief doesn't have any requires moves, so let's hop next door to the Wizard for a practical example:
Enchanter
When you have time and safety with a magic item you may ask the GM what it does, the GM will answer you truthfully.
Logical
When you use strict deduction to analyze your surroundings, you can discern realities with INT instead of WIS.
Enchanter's Soul Requires: Enchanter
When you have time and safety with a magic item in a place of power you can empower that item so that the next time you use it its effects are amplified, the GM will tell you exactly how.
Highly Logical Replaces: Logical
When you use strict deduction to analyze your surroundings, you can discern realities with Int instead of Wis. On a 12+ you get to ask the GM any three questions, not limited by the list.
Dungeon World github, "The Wizard"
It may seem like requires vs. replaces is just a question of column space; because Highly Logical replaces Logical, it has to recap the entire text of the move it's replacing if it's intended for the character to keep the benefit. Enchanter's Soul requires Enchanter, but doesn't replace it, so it can just talk about its new benefit without recapping Enchanter. But if you're designing a class and coming up with multiple moves that can work to upgrade a lower-level move, the only way to do that would be to make them all requires, unless they were intended to be mutually exclusive.
This doesn't mean the Thief is locked out of getting Dirty Fighter, however. It's easy to mentally classify class moves as "2-5 moves" and "6-10 moves", but if you look at the class it says:
When you gain a level from 6–10, choose from these moves or the level 2–5 moves.
-- Dungeon World github, "The Thief"
All of the classes say this before the 6-10 moves, even third-party ones, and if they don't you're probably safe reading them as if they did. Assuming your Thief has another level to go before 10, they can grab Cheap Shot this level and Dirty Fighting the next one.
