Currently, migrating fully invoiced billing milestones at cutover requires the creation of billing milestones and a confirmed proforma invoice. This is either a tedious manual process or requires very advanced knowledge of programmatically creating milestones and calling relevant actions to create and confirm invoices.

My suggestion is to leverage Actuals for creating Billed Sales milestones, which could then be factored in for RevRec. This would make this part of the cutover a lot faster and simpler, as the creation of Journals and Journal Lines can be done in bulk.

Migrate fully invoiced billing milestones at cutover | Microsoft Docs