I don't know if the changes were exactly at the same time as the model change - but:

GP7/SD7/SW8/SW7 have:

567B engines (square air box and crankcase covers)
The top of the draft gear pocket is a casting (rounded)
The handrails are in bolted segments between the stanchirons.

GP9/SD9/SW1200/SW900(&600)

567C engines (round air box and crankcase covers)
The draft gear pocket is fabricated (flat slope)
The top of the stanchirons wrap around a fairly continuous handrail (modern type)

Obviously, rebuilding can change things - but this is a good indication what original frame was used.

On the SP, there was also a weight difference. The SD7's and first order(s) of SD9's were around 328,000 - later SD9's were 358,000. When rebuilt, the "light" SD9's were scattered in the lower 4300s, and SP did seem to try to assign them to light rail branches (at least in Oregon).