Mergefield - Replace Prefix of Document Number

[boonlailee]

Hi, i need help on this:

Field Name: DocNumber

Sample of DocNumber (this is actually the invoice number):

INV-0001
INV-0002
INV-0003
INV-0004


I making use of this mergefield «DocNumber» to create my DeliveryOrder bearing SAME numbering, however i need it to be in different "Prefix" as per below:

DO-0001
DO-0002


DO-0003
DO-0004

Please help.
Thanks.

Boon Lai