I am on the old system still, but am looking to upgrade my phone. I understand I will be changed to the new system when I do. What I don't understand is what my bill will look like when I switch. If my bill is currently $50 and my tab charge will be $25 does that mean my bill will be $75?
Depending on the phone you get, you can put up to 360$ on the tab without changing your plan. This would result in a 15$ monthly tab charge that does in fact get added to your monthly plan, which would bring it to a total of 65$/month if you have a 50$/month plan.
And that's also assuming that your current $50 plan is better than whats currently available. You could change to one of the new plans if they are a better value and wind up paying less than the $65 Mathieu mentioned.