A codebase in an older hash format (and table layout) isn't opened, and says to migrate; `tcm migrate` moves it, history and all: its names, both branches, its history entries, its release and its test results, the old database kept beside it.
A codebase in an older table layout (`tcmts.fixtures.codebase-layout-8f648138`: patches kept whole for each branch, made by that layout's tcm) isn't opened, and says to migrate; `tcm migrate` moves its tables, keeping it as it was beside it, and each branch reads the patches it had.