A transition period rather than Stop-The-World migration; we want to merge in a few repositories per day, with minimal disruption to work-flow.

New changes to the old repositories can be imported into the monorepo and merged in. For example, in the above example, say repository one had a branch my_branch which continued to be developed after the migration. To pull those changes in:

Don't Stop The World: keep working in your other repositories during the migration and pull the changes into the monorepo as you go.