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.
The thing that makes the Local Trader address special is that its private key is used for authentication purposes (using bitcoin message signing) and for encrypting the communication with the peer you trade with (using an Elleptic Curve Diffie Hellman variant + AES & HMAC).
The address you send from or receive with depends on the currently selected address just when you send / receive funds with the wallet. So the transactions used for trading only occur with the local trader access if this is where you store your coins. You can even put the local trader address in your archive.
Once we get to HD nothing will change from a Local Trader perspective, it just uses the wallet you have with its current settings. If it happens to be a HD wallet then it will use new addresses for receiving coins and change.
Hope this clarifies