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.
It's possible to have more than one HD account
from different seeds in a single instance of Mycelium wallet.
In Accounts, click the key+ symbol, go to Advanced.
The text there says "to add a Single Address Account," but I think you can add an HD account.
There was a post by dooglus in this thread about it many pages ago.
I did it on one of my devices a while back.