Your only option is to delete the incorrect transactions and enter them correctly. The Find & recode function allows you to change posting accounts. But you cannot change the fundamental type of a transaction.
I found a workaround. So instead of deleting the transactions and re-posting which would have been Herculean.
I simply edited each transaction added a negative sign [-] to each, automatically that changed the amounts to negative in my Bank statement and also adjusted each entry line to the debit side of the expense account.
Hope that might help some else in future.