As a Customer Record can have multiple addresses; it is possible to follow one of two processes to correct this:
Simply create a new Customer Record with the correct VAT status and mark the original account to Inactive (so as to avoid users from posting any future transactions through this account).
Note: you will still be able to report on this 'Inactive' customer (this does not remove the transaction(s) and/or data from the database)
To set the original customer record to an inactive status,
A new Address can be created for this Customer Record (which an Address Record can have it's own VAT status against it)
Note: It might be worth marking the old Address Record as No longer use in order to avoid potentially using this old Address Record with the incorrect VAT.