Rewards, Add-ons, Available Funds, SIM card, and phone number are attached to the account. Changing your plan doesn't affect these other things (aside from changing the math a little).
Promo bonuses are also attached to the account, but they are sometimes conditional and usually the condition is that you must keep the promo plan you signed up with to continue getting the discount or more data or whatever bonus the promo was about. These conditions (if any) will be specified somewhere on your Self-Serve page. I think somebody once commented that the system will warn you about losing promo bonuses if you try to switch away to a non-promo plan, but this isn't something I've personally confirmed.
