I think you need change theme templates, this file: node_modules/nodebb-theme-persona/templates/header.tpl
But you need realize, in this case updates to new versions may be more painfull.
Is there any way to migrate NodeDB data from one to the other?
Not yet, right now we have import from phpBB, vB, and UBB using this strategy. Theoretically someone could write an exporter from NodeBB to NodeBB using this same strategy.