Ah, I did not understand the 60 second waiting requirement. I looked at the display again and have spotted the 60 s countdown. Had overlooked that. Thanks for the explanation.

I guess that, to create a new master seed, a new Local Trader identity, etc., I will have to move all bitcoins out of the HD wallet, uninstall Mycelium, install it again, create a new HD wallet, and restore all non-HD wallets from their backup.

Awkward, but doable. I think I now understand the motives behind this design.