Can I change my 10c/min plan to $30/month plan?

Yes, you can change to any plan online. Your balance can continue to be used to pay for any plan until it expires. Anytime you add funds to your account, you will extend the expiration date of the existing balance to match the added funds, so no reason to lose any of your existing balance to expiration. Just be aware that once you change to any plan, you can’t get that 10 cent plan back. 

@Gary L Just FYI, there is only one expiration date on the balance of a prepaid account. If the OP were to add $100, the entire $250 would be good for a year.

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@sandblaster wrote:

@Gary L Just FYI, there is only one expiration date on the balance of a prepaid account. If the OP were to add $100, the entire $250 would be good for a year.

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Yes, it would be a year from a $100 deposit (it's not stacking time with what they already have), see paragraph 2 (I even add $100 + $150 to get $250 and say 8+ months at $30 a month).

Not knowing how they deposited money, it's hard to tell (but I'd guess they've been adding $10 or $25 increments to get it up to $150).

@blackcatbowtie The issue is timing, if part of that $150 is a $100 deposit, then you could go a year (from that $100 deposit) and use the $30 a month from the $150.

If not, then you could deposit $100 to the account. That would get you a year. Then the $250 ($150+$100) could be used to pay the $30 a month for the next 8+ months.

Or you could deposit $25 to get 90 days (and pay $30 a month), but you'd have to do that every 90 days to keep the timing up.

$10-$24 payments expire in 30 days, $25 to $99 in 90 days, $100 or more in 365 days

https://www.att.com/legal/terms.prepaidPlanTerms.html