I added base plan 15 but want to change it to base plan 50. Can I do so even though base plan 15 has not run out. I also still have a talk booster left. If I change my plan now before it expires will I lose those credits?
You can... but............. unfortunately your plan will "reset" itself to that date, meaning you basically lose the remaining part of your $15 base plan.
As Dennis said, you will definitely keep your boosters but to make sure you get all the base plan days you are entitled to, you're best off to change your base plan on the last day before it renews itself.