Here we go, I found a few minutes earlier this evening. 
- I stopped Z2M (addOn) and disabled âstart at bootâ and âwatchdogâ
- shut down the rPi (just in case and I donât like hot swapping)
- pulled the old dongle (CC2531)
- pushed the new one (Slaesh) in a different USB port
- started the rPi
- waited a few minutes, so HA was def. fully started
- checked and changed the new USB port in Z2M config
- started Z2M
- waited a few minutes (read = felt hours, were seconds)
- opened Z2M dashboard and checked
- most devices were already there and online
- just a few were kind of missing (eg showed up on the map, but werenât connected in the map = no lines)
- tried a new interview with one of them, didnât change anything
- no errors up to this point, whether in Z2M log, nor in HA log(s)
- thought it is a good time to take a small walk with our puppy
- came back after around half an hour and TADA
- everything is working, reporting correct, is found on the map
So, to sum up: yep, the new âupgrade featureâ works like a charme Needless to say, always do a backup first. If you have one, you wonât need it, if you have none, youâll want it! 