You’ll want to get a copy of 2.6 (see 6.5.2). There have been a lot of changes to DG1. Fields that initially were deprecated in 2.3 have been removed from the standard, and others have been added. As far as backward compatibility goes, field numbers have not changed. The fields are just no longer in use
The HL7 standard is copyrighted. You can get it by you or your organization joining HL7 or from a vendor licensed to distribute it.
For those who would like to have a more in depth understanding of HL7, both V2 and V3, the next round of HL7 eLearning will be March 11 to June 24. More information is available on their website.