ReadonlyfromThe version of the document handed to the step.
ReadonlytoThe version the step's result is stamped with — always fromVersion + 1.
Reports a lossy change (§80). Purely informational: the message reaches the caller through MigrateSceneDocumentOptions.onWarning and nothing in the engine reads it, so it should say what was dropped and what replaced it.
What a migration step is told about the link it is performing (§80).