How is/was RFID bag handling introduced in a baggage handling system tailored to barcodes?
By printing them along with bar codes. RFID mode became operational in HK in 2005. Basically, the airport replaced the barcode only baggage tags with RFID embedded tags:


  Hong Kong International Airport (HKIA) today announced that all of its check-in counters can now print integrated radio frequency identification (RFID) baggage tags. The new tags, which combine an embedded RFID chip with a barcode, are replacing barcode-only baggage tags on an airline-by-airline basis.


Basically, the system reads both the RFID tags and barcodes.