Thanks Jeff ( @jtp10181 )
I was able to update the firmware on two Zen 72 dimmers (to firmware 2.20), and a Zen 54 0-10 volt module (to firmware 1.30) using a Zooz Zstick but this did not resolve some weird routing issues where one of the Zen72 switches was going direct to hub, but the other (in the same 2 gang box) was routing through two other zwave devices.
Setting the Priority route to 00 00 00 00 for the problematic Zen72 device fixed the issue after I set it, then removed the Zstick. Now both route directly to the hub at 100 kbps 
These two dimmers control PTC inline heaters in my HRV setup, so having them worked correctly together is important:
