Yes the discount rate TheoTrade has with TD Ameritrade does apply to existing accounts. If you need open a new TD account you can get get 60 days of free trading by opening an account using this link:
The discount rate is exceptional therefore TD does not allow us to publicly display the discount rate. You can email email@example.com to find out the discount rate.
In order to get the discount rates TD requires that you become a TheoTrade member. Here are the membership options:
Or you can get one of these TD approved classes:
If you already have a TD account as soon as you become a TheoTrade member you will be sent instructions on how to request the discount commission rate. It typically takes 1 business day to get the discount commission applied to your account.