The reason for simulcasting 107.5 on 97.5 is the few miles on I-85 where Radio One's 107.9 completely blocks out 107.5.
Here's my guess as to what Radio One is planning: The Classic Hip-Hop format moves from 102.9 to 97.5, giving the format a full FM signal and a leg up on Cumulus' and Steve Hegwood's Classic Hip-Hop translator signals. Then, Radio One uses 102.9 to fill in the 107.5 signal in the area when it's blocked by 107.9.
The 2 obvious things that would be accomplished would be: 1)The 97.5 signal would be freed up to give R1's Classic Hip-Hop format the competitive advantage of being on a full FM. 2) Radio One wouldn't have to waste a full FM signal on the small area where 107.5 cannot be picked up; a less-valuable translator would be assuming that role.