Currently, this process will only affect Java files.

Say you want to migrate from 1.14.4 to 19w46b.

1. Go here, select 19w46b, and copy the yarn_mappings value, for example 19w46b+build.1. DO NOT modify your gradle.properties or build.gradle yet.
2. In the root of your gradle project, run gradlew migrateMappings —-mappings “19w46b+build.1”
3. Your migrated source will appear in remappedSrc. Verify your code was not nuked and copy it over.
4. Update your gradle.properties file according to the instructions in this site.
5. You're done.

• Specify from where to take your Java files with —-input path/to/source. Default: src/main/java.
• Specify where to output the remapped source with —-output path/to/output. Default: 'remappedSrc'.
• Specify a custom place to retrieve the mappings from with —-mappings some_group:some_artifact:some_version:some_qualifier. Default: net.fabricmc:yarn:<version-you-inputted>:v2.

• Fabric-Loom 0.2.2 or above
• A Java codebase - Kotlin and Scala will not, currently, do.
• Some assembly required

1. Figure out your target mappings version. For example, “net.fabricmc:yarn:1.14.1 Pre-Release 2+build.2”.
2. Make sure the mappings for this version get created. This is the hacky part, as currently the only way to do it is to edit the “minecraft” and “mappings” fields in a build.gradle to the new version, run any Gradle command (“gradle build” will do, even if it crashes), then change the fields back.
3. Run the following magical wizardry command: gradle migrateMappings -PtargetMappingsArtifact="net.fabricmc:yarn:1.14.1 Pre-Release 2+build.2" -PinputDir=src/main/java -PoutputDir=remappedSrc, where:
   • “targetMappingsArtifact” refers to the target mappings version. It is imperative that the build.gradle be set to the current mappings version of the mod when running this command!
   • “inputDir” is the input directory, containing Java source code,
   • “outputDir” is the output directory. It will be created if it is missing.
4. Copy the remapped source code to the input directory, if everything's fine,
5. Hope for the best.

Note: You may need to specify the full paths in quotes, try this if you get file not found issues.

This should work across Minecraft versions as well, provided we haven't massively broken Intermediaries or done something equally silly (aka: most of the time).