When migrating from a single instance to OneWorld, does NetSuite provide an entirely new NetSuite instance or do we get to retain our transactions and everything else but has now the features that are available to OneWorld accounts?

I believe everything stays as is until you add a subsidiary

You keep transactions and everything else under the parent subsidiary that will be automatically created

Oh no. If use currently use custom segments for the child subsidiaries, at a high level, how can we properly migrate to OneWorld using the same NS account? As such, every transaction and record is associated with the parent subsidiary. I would assume that since the prior periods are already closed then we cannot do anything with that anymore. We just need to move forward by creating the child subsidiaries and migrating over their open transactions from custom segments to child subsidiaries?

Isnt it also an option to have the Original NS become Child A, and the new one become Child B? And Parent is just a umbrella?

When we transition to OneWorld, will we have an option to define a new parent for Original NS entity like that dummy one that you are talking about?