Per standard Netsuite functionality, it is not allowed to change the currency setup once a transaction has been saved.
Below is an alternative solution to this.
If the currency is incorrect, we can make a copy of the transaction and change the currency on the copy instead.
To do this, open the invoice record and on Click the dropdown under Actions button > Select Make Copy. On the copy of the invoice, In the Currency field, select the appropriate currency to be used for the transaction.
In case we do not see the currency that we need under the currency field, check whether this currency has not been set up yet for this customer. We can set this up on the Financial subtab of the customer record.