Substring Replacement

Substring Replacement is a string transformation used in certain word-based puzzles and metapuzzles, wherein a particular substring (letters, words, phrases, etc.) is replaced with a different string (often related to it). Alternatively, it can also refer to puzzles wherein a particular substring has been replaced prior to solving, and identification of what it was is required.

Puzzle Application
The most common use of substring replacement is as a prelude to extraction, such that a particular word or phrase can fit a certain length or series of blanks that it otherwise could not. Other uses include replacing a common word or type of word with a single letter prior to use in a more jumble-based puzzle, or providing a list of substring replacements to apply multiple transformation

The relationship between the substrings and what they are replaced by can also vary. In some cases, the replacement will be directly related to the string, such as replacing a word with a symbol that represents it. At other times, particularly when extraction comes soon after the replacement, a string will be replaced by a completely unrelated letter (or word, etc.) that is only deemed important due to its use in the extraction.

While certain substring replacement puzzles mandate that the resulting string still be a valid word or phrase, this is not a rule for this type of puzzles, and many puzzles will still require the creation of nonsense phrases for the sake of a consistent mechanic.

Strategy
Depending on the type of substring that a puzzle is focusing on, the difficulty in identifying the substring to replace (or that has been replaced) can range from simple to middling. If a common substring has been replaced already (like a common word in song titles), or the substrings are otherwise large, common words, it should be relatively obvious what one is dealing with. Common tactics for making the identification portion of a substring replacement step more difficult include splitting a substring with a space, using shorter substrings within longer parent strings, and using multiple different substrings that are related to each other.

For the transformation aspect of this, solvers should take into consideration what context they've been given. If they have enumerations or blanks that don't fit the provided strings, they should try to identify thematic transformations that would make them fit. If they only have indexes, there will likely be some other clues (either within the title/flavortext or in an extra aspect of the puzzle) that will help figure out the next step.

NOTE: For puzzles where a transformation has already taken place, and the results are presented, often only the identification step will be needed for solving.