Nooo you didn't do anything dumb at all :) your next bill will be prorated, meaning 3 weeks will be billed according to the old rate and 1 week with the new rate. I would imagine you'll hardly notice it!
Your minutes will be prorated. If you used up your minutes and changed it, then you might have minutes overage charge. What plan did you have and how many minutes you used before you changed your plan? We might be able to give you better number for your next bill.