Correct answers
No, they didn't mess up - but the rep COULD have been kind enough to tell you about pro-rating. (Unless you did this online--and apparently the option is not there when you do it that way.) 
What happened is you were "pro-rated" the allowance for the first part of the month based on 1500 texts (or about 50 texts per day for however many days you were on the 1500 plan), then the allowance increased to about 150 or so a day for the balance of the month. So if you were already over 1500 at the halfway point, then pro-rating is going to result in overages.
What I would do is call CS, and politely, calmly explain that you were never given the opportunity to BACKDATE the plan to the beginning of the billing cycle to cover the overages, so could they PLEASE make the new plan effective for the entire previous billing cycle? That should cover all but the 100 messages over THAT allowance, which you said is all there was over the 5000. Or better yet, just go to the new Tak & Text plan, which is only $20 over the basic Talk plan and you pay that for the 5000 messages anyway, and you'd have unlimited with no worries about overages. Again, ask nicely, to make the texting plan effective back to the first of the previous billling cycle, and then get yourself on an unlimited plan....
No, they didn't mess up - but the rep COULD have been kind enough to tell you about pro-rating. (Unless you did this online--and apparently the option is not there when you do it that way.)
What happened is you were "pro-rated" the allowance for the first part of the month based on 1500 texts (or about 50 texts per day for however many days you were on the 1500 plan), then the allowance increased to about 150 or so a day for the balance of the month. So if you were already over 1500 at the halfway point, then pro-rating is going to result in overages.
What I would do is call CS, and politely, calmly explain that you were never given the opportunity to BACKDATE the plan to the beginning of the billing cycle to cover the overages, so could they PLEASE make the new plan effective for the entire previous billing cycle? That should cover all but the 100 messages over THAT allowance, which you said is all there was over the 5000. Or better yet, just go to the new Tak & Text plan, which is only $20 over the basic Talk plan and you pay that for the 5000 messages anyway, and you'd have unlimited with no worries about overages. Again, ask nicely, to make the texting plan effective back to the first of the previous billling cycle, and then get yourself on an unlimited plan....