An update:

I was getting close to running out, so I went ahead and added enough credit for the 30-day package. I then tried to activate it online (mitelcel.com) but was told I could not activate it until 24 hours before the end of my current package (I was on day 2 of 7). I then tried to activate it by text ("alto30") and was told I could reactivate my current plan after I exhausted my current data.

Once my data ran out, I got a text and immediately sent "alto30". I got an error popup saying I didn't have enough credit, but then I got a text saying my 3GB package had been activated.

So, it's a little tricky to time it, but it worked. I just didn't want my current plan to auto-renew or to get charged the per-KB data rate after using all of my previous plan (which is why I was hesitant to just add credit without also confirming the new data plan), but it looks like that didn't happen.

Also, for folks looking for more details on this topic in the future, please also reference this thread: http://www.flyertalk.com/forum/mexic...ns-mexico.html It looks like it's monitored a bit better than this one.