It depends, which FW 74 is installed. From .200 and .230 a downgrade is possible.
If you installed .310 or higher you can only downgrade as low as .310. See also my post here:
NEW 74 Mazda CONNECT MZD Firmware ,CP/AA, WCP, links, Help
In general Mazda does not recommend a downgrade to FW 70 or lower if your CMU came from factory with FW 74.
Technically there is a complicated way to downgrade even from .310 or .311 to FW 70 - see here:
Hi sir, do you have any video to show? No I don't, Sorry. I tried it though anyways and it worked flawlessly. My car is alive again!!!
www.mazda3revolution.com
And yes, with the new hub you can downgrade to FW 70 and the hub still works. But do not downgrade to FW 59 or lower.