I am on one of the old (pre-April 2015) plans, 60$ per month, with 2 Gb data. My tab is positive (I could use it, for example, to upgrade my current iPhone). It isn't clear what happens to my positive tab if I switch to a newer plan (60$ per month, 5 Gb data) when I use the self-serve. Can anyone explain what happens to the positive tab? Does it disappear? Thanks for your feedback!