In standard MIDI, a bank change consists of one or two numbers -- the MSB and LSB. If your Korg device refers to banks as letters, you will have to find out how it maps standard bank MSB and LSB numbers to those letters. In other words, what MSB and LSB numbers do you have to send to select a given bank. This should be in the Korg manual (which you should be able to find online), or you can search Korg support forums or contact Korg tech support.