"Difficult part first" is the right approach for legacy migration, or any mandated enterprise dev, to surface tricky issues ASAP. MVP is about product market fit, which is a given via mgmt authority in a corp environment.
Compexity comes sometimes from not so important features that could take much longer to migrate. But of course some of these features are why some products are picked over others in the first place, however this requires a case study and more analysis.