I've been long wanting to do this test, and today finally got around to it with a few spare CPU trays and Gen8 and Gen9 chassis right next to each other.
Unfortunately, it seems that:
1. You can't boot a V3 CPU with DDR3 memory on an HP U17 BIOS. It complains about mismatched memory vs. CPU. This is silly, since the memory boards are interchangeable and the hardware fully supports it. HP seems to just disallow this to increase market segmentation and thereby profits.
2. You can't boot a V2 CPU + DDR3 on a U17 BIOS.
3. You can't boot a V3 CPU + DDR3 on a P79 BIOS.
4. You can't flash a U17 BIOS on a DL580 G8 with a P79 BIOS.
Basically, despite the CPU socket being exactly the same for E7-V2, E7-V3 and E7-V4, and the CPU trays being swappable, and the CPU trays and motherboards being identical, HP has firmware locked out all of the neat things this hardware platform was designed to be able to do in terms of interchangeability and scalability. What a shame. :/
Unfortunately, it seems that:
1. You can't boot a V3 CPU with DDR3 memory on an HP U17 BIOS. It complains about mismatched memory vs. CPU. This is silly, since the memory boards are interchangeable and the hardware fully supports it. HP seems to just disallow this to increase market segmentation and thereby profits.
2. You can't boot a V2 CPU + DDR3 on a U17 BIOS.
3. You can't boot a V3 CPU + DDR3 on a P79 BIOS.
4. You can't flash a U17 BIOS on a DL580 G8 with a P79 BIOS.
Basically, despite the CPU socket being exactly the same for E7-V2, E7-V3 and E7-V4, and the CPU trays being swappable, and the CPU trays and motherboards being identical, HP has firmware locked out all of the neat things this hardware platform was designed to be able to do in terms of interchangeability and scalability. What a shame. :/