I was looking for information for a client and also found a post on Community asking for help with changing Debtor ID’s.
After some investigation I found that from Dynamics GP 2013, PSTL no longer has the ability to edit Debtor ID’s since this is now built in to Dynamics GP as the ‘Customer Combiner and Modifier’.
You can get to this functionality by going to Sales >> Utilities >> Customer Combiner and Modifier.
Once here, you can select Customer Modifier and fill out the table with the left hand column with the current ID and the right hand column with your new ID.
Alternatively if you would like to import your changes to bulk change the ID’s you can use a CSV file with column A as the old ID and column B as the new ID.