You cannot change the local trader address associated with a trader account.
The encrypted trade history of your account can only be decrypted with the private key of the address the trader account is associated with.
In the wallet, one has the advanced options, beyond making all transactions on a single address.
As you continue developing mycelium's capabilities, do you foresee enabling more advanced functionality in local trader?
I aspire to building a reputation as a trader, and I wish to make many transactions in the coming years. Making all the transactions at the same address brings up privacy concerns.
As you work to integrate HD in the wallet, in the same vein, please consider making improvements in the flexiblity of local trader; perhaps syncing local trader with the main wallet's HD, for example.