Agree with soccer_dan 100%. The newer BIOS versions have included the older microcode, and older listed processors can still work fine with the newer BIOS, including even with the newer motherboard version detailed below.
I'm hoping you know that there are two versions of these workstations (Z420, Z620, Z820) and that the newer versions are the ones that can run the v2 processors. HP calls those newer versions "revision 1" and the easiest way to tell if your Z420 is the newer version is to look in BIOS at the "Boot block date". If yours is 12/28/11 you are out of luck, and if it is 3/6/13 you can run the v2 processors.
It is unfortunate that in the QuickSpecs for these workstations HP does not clarify this issue...... the newer QuickSpecs ignore the original version, and I guess they expect you to magically know this stuff and go to the older QuickSpecs for info on the original release.
There is no way to upgrade your Z420 to a revision 1 workstation if you have an original, unless you do a motherboard transplant. Here's the two pictures for original Z420, versus revision 1. I don't have the dates for the Z620 or Z820 handy right now:
What you got?