For reference the 6TB drive for a 7K is K0F28A with a higher list price, but the 8K drive K2P96B is priced lower. Note the 7K is 6gb SAS loop, while the 8K is a 12gb loop. The 8K introduced the new all inclusive licensing model, which the 7K requires to buy additional licenses based on number of drives. If you were buying upgrades from HPe, they would have to use the 7K sku so that the quoting tools would add the proper software and support required from a 7k.
One question I would ask is: Does HPe test/validate 12g SAS drives running in a 7K 6g shelf? Do they currently require a failed 6g drives be replaced with a 6g drive, or do they permit a newer 12g drive to be a replacement part for a failed 6g drive?
Can you elaborate how they were confirmed to be completely compatible?
Check this service out, its for rebalancing, to include moving drives between arrays, but you will need to work with your sales team to see if what you are asking is eligible:
https://h20195.www2.hpe.com/V2/GetPDF.a ... 280ENW.pdf