I am trying to update the default payables account on a batch of suppliers matching by internal ID. Simple 2 column import internal id and default payables a/c. The update say's it is successful but the data has not changed. Any ideas why?

try one record, and hard set the Default Payables a/c to see if that works. Additionally, I would also try using the Internal ID for the Default Payables account. Although we expect there to be a fail, is the Account available to that Subsidiary?