Migration between platforms, regardless it is marketing automation, CRM, ERP or anything else goes far beyond moving data.
Porting the existing processes, training, designing new ones to take full advantage of all features are vital to successful migrations. I would suggest actively working with Marketo Professional Services since the start.
For the record and to ensure some impartiality, that is not different in nature from migration between ERP platforms I was involved in the past. There are very successful and very miserable stories of movements between Oracle and SAP.