Dave, for comparison consider the BAe 146 / Avro RJ. The initial certification for the 146-200 / RJ85 at LCY (actually a general steep approach capability at any airport) was not acceptable for the 146-300 / RJ100 because of small differences in aircraft capability during the approach, even though the aircraft have the same type certificate.
The certification margins for a steep approach are small. Several aircraft types have had to make physical changes or involve new operating technique in order to comply.
The larger 146/RJ aircraft certification differed in approach angle and tailwind approval (6 deg - 5.5 deg, 5 kts - 0 kts); thus if the 7x was near or at a limit, then recertification for the 8x would be understandable.