Maybe keep happy hour the same but add just one (maybe their choice of) premium feature for an extended period of time... a month, say. I'd think private message is the best premium feature... after one month (or whatever time frame you decide) it goes away if they don't subscribe.
Matt.you may want to think about your broker bizz and figure some thing to work with with the free ones that would make them want to join sooner after your sure of every thing. I do feel that you are giving them way to much room with what you have here.
Market Data 
Markets 
AI
