It’s not a reporting problem, it’s a data problem. Customers are hard-coded to belong to a single subsidiary. You would need to recreate the customer under the new sub and then possibly migrate some of their financial history if desired, and then start using that new customer from now on.
If you are patient, NetSuite will eventually allow a single customer to belong to multiple subsidairies. However, in your case, I’m not sure even that would help, since their historical data would still belong to the old sub. There’s nothing you can ever do to resolve that, short of migrating your data from sub to sub, or perhaps using a custom field to reclassify the data (though that would be difficult to manage on the long run)