I tried renewing my permit. I can tell you this:
1) You have to renew at the frontier (border) - Aduana offices inside the country (Guatemala City, Xela, etc.) won't handle the renewal.
2) You will have no problem if you return to your original border entry crossing.
3) If the Aduana computer system is down, you can't renew unless you do #2.
I know #2 and #3 because I attempted to renew on a Saturday, travelling 125km from Xela to the nearest Mexican border crossing point only to find out their computer system was down and I was told I needed to return the next day. If I had returned to my original entry point, they could pull my paperwork and update the computer files when the system became available.
Chiang Mai, Thailand